diff --git a/.planning/ROADMAP.md b/.planning/ROADMAP.md
index d557ef0..087a1b8 100644
--- a/.planning/ROADMAP.md
+++ b/.planning/ROADMAP.md
@@ -74,7 +74,10 @@ Plans:
   1. All chart fills (donut slices, bar segments) use the semantic category colors from `lib/palette.ts` — "Bills" is the same color in the donut chart as it is in the FinancialOverview table
   2. Chart tooltips display values formatted with the budget's currency (e.g., "1,234.56" not "1234.56")
   3. The `formatCurrency` function uses the user's locale preference from their settings instead of the hardcoded `de-DE` — an English-locale user sees their numbers formatted correctly
-**Plans**: TBD
+**Plans:** 2 plans
+Plans:
+- [ ] 04-01-PLAN.md — TDD: formatCurrency locale parameter fix (FIX-01)
+- [ ] 04-02-PLAN.md — Chart tooltip wiring and locale threading (IXTN-04)
 
 ## Progress
 
@@ -86,4 +89,4 @@ Phases execute in numeric order: 1 -> 2 -> 3 -> 4
 | 1. Design Token Foundation | 2/2 | Complete   | 2026-03-11 |
 | 2. Layout and Brand Identity | 0/2 | In progress | - |
 | 3. Interaction Quality and Completeness | 0/4 | Not started | - |
-| 4. Chart Polish and Bug Fixes | 0/TBD | Not started | - |
+| 4. Chart Polish and Bug Fixes | 0/2 | Not started | - |
diff --git a/.planning/phases/04-chart-polish-and-bug-fixes/04-01-PLAN.md b/.planning/phases/04-chart-polish-and-bug-fixes/04-01-PLAN.md
new file mode 100644
index 0000000..7ce5759
--- /dev/null
+++ b/.planning/phases/04-chart-polish-and-bug-fixes/04-01-PLAN.md
@@ -0,0 +1,103 @@
+---
+phase: 04-chart-polish-and-bug-fixes
+plan: 01
+type: tdd
+wave: 1
+depends_on: []
+files_modified:
+  - frontend/src/lib/format.ts
+  - frontend/src/lib/format.test.ts
+autonomous: true
+requirements:
+  - FIX-01
+
+must_haves:
+  truths:
+    - "formatCurrency no longer hardcodes 'de-DE' — the default locale is 'en'"
+    - "formatCurrency accepts an optional third locale parameter"
+    - "Calling formatCurrency(1234.56, 'EUR', 'de') produces German-formatted output"
+    - "Calling formatCurrency(1234.56, 'USD', 'en') produces English-formatted output"
+    - "Calling formatCurrency(1234.56, 'EUR') without locale uses 'en' default, not 'de-DE'"
+  artifacts:
+    - path: "frontend/src/lib/format.ts"
+      provides: "Locale-aware formatCurrency function"
+      contains: "locale"
+    - path: "frontend/src/lib/format.test.ts"
+      provides: "Unit tests for formatCurrency locale behavior"
+      min_lines: 20
+  key_links:
+    - from: "frontend/src/lib/format.ts"
+      to: "Intl.NumberFormat"
+      via: "locale parameter passed as first arg"
+      pattern: "Intl\\.NumberFormat\\(locale"
+---
+
+<objective>
+Fix the hardcoded `'de-DE'` locale in `formatCurrency` by adding an optional `locale` parameter with `'en'` as the default. This is the foundation for IXTN-04 (chart tooltips) and ensures all currency formatting respects the user's locale preference.
+
+Purpose: FIX-01 — English-locale users currently see German number formatting everywhere
+Output: Updated `format.ts` with locale parameter, comprehensive unit tests
+</objective>
+
+<execution_context>
+@/home/jean-luc-makiola/.claude/get-shit-done/workflows/execute-plan.md
+@/home/jean-luc-makiola/.claude/get-shit-done/templates/summary.md
+</execution_context>
+
+<context>
+@.planning/PROJECT.md
+@.planning/ROADMAP.md
+@.planning/phases/04-chart-polish-and-bug-fixes/04-RESEARCH.md
+@frontend/src/lib/format.ts
+</context>
+
+<feature>
+  <name>Locale-aware formatCurrency</name>
+  <files>frontend/src/lib/format.ts, frontend/src/lib/format.test.ts</files>
+  <behavior>
+    - formatCurrency(1234.56, 'EUR', 'en') returns a string containing "1,234.56" (English grouping)
+    - formatCurrency(1234.56, 'EUR', 'de') returns a string containing "1.234,56" (German grouping)
+    - formatCurrency(1234.56, 'USD', 'en') returns a string containing "$" and "1,234.56"
+    - formatCurrency(1234.56, 'EUR') with NO locale arg uses 'en' default — does NOT produce German formatting
+    - formatCurrency(0, 'EUR', 'en') returns a string containing "0.00"
+    - formatCurrency(-500, 'EUR', 'en') returns a string containing "-" and "500.00"
+    - formatCurrency(1234.56, 'EUR', '') falls back gracefully (does not throw RangeError)
+  </behavior>
+  <implementation>
+    Update `frontend/src/lib/format.ts`:
+
+    ```ts
+    export function formatCurrency(
+      amount: number,
+      currency: string = 'EUR',
+      locale: string = 'en'
+    ): string {
+      return new Intl.NumberFormat(locale || 'en', {
+        style: 'currency',
+        currency,
+      }).format(amount)
+    }
+    ```
+
+    Key points:
+    - Third parameter `locale` defaults to `'en'` (replacing hardcoded `'de-DE'`)
+    - Defensive `locale || 'en'` guards against empty string (which throws RangeError)
+    - All existing call sites pass only 2 args — they will now get English formatting instead of German
+    - This is intentional and correct per FIX-01
+  </implementation>
+</feature>
+
+<verification>
+cd frontend && bun vitest run src/lib/format.test.ts
+</verification>
+
+<success_criteria>
+- format.test.ts has at least 6 test cases covering locale variations, defaults, and edge cases
+- All tests pass green
+- formatCurrency signature has 3 parameters: amount, currency, locale
+- No hardcoded 'de-DE' remains in format.ts
+</success_criteria>
+
+<output>
+After completion, create `.planning/phases/04-chart-polish-and-bug-fixes/04-01-SUMMARY.md`
+</output>
diff --git a/.planning/phases/04-chart-polish-and-bug-fixes/04-02-PLAN.md b/.planning/phases/04-chart-polish-and-bug-fixes/04-02-PLAN.md
new file mode 100644
index 0000000..9f7dda7
--- /dev/null
+++ b/.planning/phases/04-chart-polish-and-bug-fixes/04-02-PLAN.md
@@ -0,0 +1,219 @@
+---
+phase: 04-chart-polish-and-bug-fixes
+plan: 02
+type: execute
+wave: 2
+depends_on:
+  - "04-01"
+files_modified:
+  - frontend/src/pages/DashboardPage.tsx
+  - frontend/src/components/ExpenseBreakdown.tsx
+  - frontend/src/components/AvailableBalance.tsx
+autonomous: true
+requirements:
+  - IXTN-04
+
+must_haves:
+  truths:
+    - "Hovering over an ExpenseBreakdown pie slice shows a tooltip with the category name and currency-formatted value"
+    - "Hovering over an AvailableBalance donut slice shows a tooltip with the segment name and currency-formatted value"
+    - "Chart tooltip values use the budget's currency code (not hardcoded EUR)"
+    - "Chart tooltip values use the user's preferred_locale for number formatting"
+    - "AvailableBalance center text also receives the user's locale for formatting"
+  artifacts:
+    - path: "frontend/src/components/ExpenseBreakdown.tsx"
+      provides: "Custom Recharts Tooltip with formatted currency"
+      contains: "formatCurrency"
+    - path: "frontend/src/components/AvailableBalance.tsx"
+      provides: "Custom Recharts Tooltip on donut chart + locale-aware center text"
+      contains: "Tooltip"
+    - path: "frontend/src/pages/DashboardPage.tsx"
+      provides: "Locale threading from useAuth to chart components"
+      contains: "useAuth"
+  key_links:
+    - from: "frontend/src/pages/DashboardPage.tsx"
+      to: "frontend/src/hooks/useAuth.ts"
+      via: "useAuth() hook call to get user.preferred_locale"
+      pattern: "useAuth\\(\\)"
+    - from: "frontend/src/pages/DashboardPage.tsx"
+      to: "frontend/src/components/ExpenseBreakdown.tsx"
+      via: "locale prop passed to ExpenseBreakdown"
+      pattern: "locale=.*userLocale"
+    - from: "frontend/src/pages/DashboardPage.tsx"
+      to: "frontend/src/components/AvailableBalance.tsx"
+      via: "locale prop passed to AvailableBalance"
+      pattern: "locale=.*userLocale"
+    - from: "frontend/src/components/ExpenseBreakdown.tsx"
+      to: "frontend/src/lib/format.ts"
+      via: "formatCurrency called inside Tooltip content renderer"
+      pattern: "formatCurrency.*budget\\.currency.*locale"
+    - from: "frontend/src/components/AvailableBalance.tsx"
+      to: "frontend/src/lib/format.ts"
+      via: "formatCurrency called inside Tooltip content renderer and center text"
+      pattern: "formatCurrency.*budget\\.currency.*locale"
+---
+
+<objective>
+Wire custom currency-formatted tooltips into both chart components and thread the user's locale preference from `useAuth` through `DashboardPage` to the charts. After this plan, hovering over any chart segment shows a properly formatted currency value.
+
+Purpose: IXTN-04 — chart tooltips currently show raw numbers without currency formatting
+Output: Both charts have styled tooltips, locale flows from user settings to chart display
+</objective>
+
+<execution_context>
+@/home/jean-luc-makiola/.claude/get-shit-done/workflows/execute-plan.md
+@/home/jean-luc-makiola/.claude/get-shit-done/templates/summary.md
+</execution_context>
+
+<context>
+@.planning/PROJECT.md
+@.planning/ROADMAP.md
+@.planning/phases/04-chart-polish-and-bug-fixes/04-RESEARCH.md
+@.planning/phases/04-chart-polish-and-bug-fixes/04-01-SUMMARY.md
+
+<interfaces>
+<!-- Key types and contracts the executor needs -->
+
+From frontend/src/lib/format.ts (after Plan 01):
+```typescript
+export function formatCurrency(amount: number, currency?: string, locale?: string): string
+```
+
+From frontend/src/lib/api.ts:
+```typescript
+export interface User {
+  // ...
+  preferred_locale: string
+  // ...
+}
+
+export interface BudgetDetail {
+  id: string
+  name: string
+  currency: string
+  // ...
+  totals: BudgetTotals
+  items: BudgetItem[]
+}
+```
+
+From frontend/src/hooks/useAuth.ts:
+```typescript
+export function useAuth(): {
+  user: User | null
+  loading: boolean
+  login: (email: string, password: string) => Promise<void>
+  // ...
+}
+```
+
+From frontend/src/components/ExpenseBreakdown.tsx (current):
+```typescript
+interface Props {
+  budget: BudgetDetail
+}
+// Needs locale prop added
+```
+
+From frontend/src/components/AvailableBalance.tsx (current):
+```typescript
+interface Props {
+  budget: BudgetDetail
+}
+// Needs locale prop added, also needs Tooltip import added
+```
+
+From frontend/src/lib/palette.ts:
+```typescript
+export type CategoryType = 'income' | 'bill' | 'variable_expense' | 'debt' | 'saving' | 'investment' | 'carryover'
+export const palette: Record<CategoryType, { light: string; medium: string; base: string }>
+```
+</interfaces>
+</context>
+
+<tasks>
+
+<task type="auto">
+  <name>Task 1: Add locale prop and custom Tooltip to both chart components</name>
+  <files>frontend/src/components/ExpenseBreakdown.tsx, frontend/src/components/AvailableBalance.tsx</files>
+  <action>
+    **ExpenseBreakdown.tsx:**
+    1. Add `locale?: string` to the `Props` interface
+    2. Import `formatCurrency` from `@/lib/format`
+    3. Replace bare `<Tooltip />` with a custom `content` renderer:
+       ```tsx
+       <Tooltip
+         content={({ active, payload }) => {
+           if (!active || !payload?.length) return null
+           const item = payload[0]
+           return (
+             <div className="rounded-lg border border-border/50 bg-background px-2.5 py-1.5 text-xs shadow-xl">
+               <p className="font-medium text-foreground">{item.name}</p>
+               <p className="font-mono tabular-nums text-muted-foreground">
+                 {formatCurrency(Number(item.value), budget.currency, locale)}
+               </p>
+             </div>
+           )
+         }}
+       />
+       ```
+    4. Destructure `locale = 'en'` from props with default
+
+    **AvailableBalance.tsx:**
+    1. Add `locale?: string` to the `Props` interface
+    2. Import `Tooltip` from `recharts` (add to existing import)
+    3. Add `<Tooltip>` with identical custom `content` renderer pattern inside the `<PieChart>` after the `<Pie>` element
+    4. Update the center text `formatCurrency(available, budget.currency)` call to include locale: `formatCurrency(available, budget.currency, locale)`
+    5. Destructure `locale = 'en'` from props with default
+
+    **Do NOT** use `ChartContainer` or `ChartTooltipContent` from shadcn — these charts use raw Recharts primitives and the project rule forbids editing shadcn ui source files.
+
+    **Tooltip styling** must match shadcn design system: `rounded-lg border border-border/50 bg-background px-2.5 py-1.5 text-xs shadow-xl` — this replicates the ChartTooltipContent styling without importing it.
+  </action>
+  <verify>
+    <automated>cd /home/jean-luc-makiola/Development/projects/SimpleFinanceDash/frontend && bun vitest run</automated>
+  </verify>
+  <done>Both chart components accept an optional locale prop, render custom tooltips with formatCurrency, and AvailableBalance center text passes locale to formatCurrency</done>
+</task>
+
+<task type="auto">
+  <name>Task 2: Thread user locale from useAuth through DashboardPage to chart components</name>
+  <files>frontend/src/pages/DashboardPage.tsx</files>
+  <action>
+    1. Add `useAuth` import: `import { useAuth } from '@/hooks/useAuth'`
+    2. Inside `DashboardPage` function body, call: `const { user } = useAuth()`
+    3. Derive locale with defensive fallback: `const userLocale = user?.preferred_locale || 'en'`
+    4. Pass `locale={userLocale}` prop to both chart component instances:
+       - `<AvailableBalance budget={current} locale={userLocale} />`
+       - `<ExpenseBreakdown budget={current} locale={userLocale} />`
+    5. Do NOT pass locale to other components (BillsTracker, VariableExpenses, DebtTracker, FinancialOverview) — those components use formatCurrency with the new 'en' default which is correct. A full locale-threading pass across all table components is out of scope for this phase.
+
+    The `useAuth()` hook is idempotent — it reads from the same React state already initialized by `App.tsx`, so there is no double-fetch concern.
+  </action>
+  <verify>
+    <automated>cd /home/jean-luc-makiola/Development/projects/SimpleFinanceDash/frontend && bun vitest run</automated>
+  </verify>
+  <done>DashboardPage calls useAuth(), derives userLocale, and passes it to both ExpenseBreakdown and AvailableBalance as a locale prop</done>
+</task>
+
+</tasks>
+
+<verification>
+1. `cd frontend && bun vitest run` — full test suite passes
+2. `cd frontend && bun run build` — production build succeeds with no TypeScript errors
+3. Manual: hover over ExpenseBreakdown pie slices — tooltip shows category name + formatted currency
+4. Manual: hover over AvailableBalance donut slices — tooltip shows segment name + formatted currency
+5. Manual: AvailableBalance center text formats using user's locale preference
+</verification>
+
+<success_criteria>
+- ExpenseBreakdown shows formatted currency tooltip on hover (not raw numbers)
+- AvailableBalance shows formatted currency tooltip on hover (previously had no tooltip)
+- AvailableBalance center text uses the user's locale for formatting
+- DashboardPage reads user.preferred_locale via useAuth and threads it to both chart components
+- Full test suite and build pass with no errors
+</success_criteria>
+
+<output>
+After completion, create `.planning/phases/04-chart-polish-and-bug-fixes/04-02-SUMMARY.md`
+</output>