diff --git a/SENDER_EDITOR_WASM_PDF_EDITOR_IMPLEMENTATION_PLAN_en.md b/SENDER_EDITOR_WASM_PDF_EDITOR_IMPLEMENTATION_PLAN_en.md
index dc8ee9d3..d645f2fa 100644
--- a/SENDER_EDITOR_WASM_PDF_EDITOR_IMPLEMENTATION_PLAN_en.md
+++ b/SENDER_EDITOR_WASM_PDF_EDITOR_IMPLEMENTATION_PLAN_en.md
@@ -11,6 +11,7 @@ This plan is prepared to remove the DevExpress PDF viewer dependency from `Envel
 - **Library constraint**: `pdf.js` and DevExpress PDF viewer will not be used. For PDF rasterization, first verify whether DevExpress provides a suitable capability; if not, use a reliable, popular MIT-licensed alternative.
 - **Server-side pattern rule**: All new server-side work must follow the existing architecture; **Clean Architecture layers** and **MediatR pattern** must be preserved.
 - **Language rule (code side)**: All code and any code comments must always be in English.
+- **Localization rule**: UI text must not be hardcoded; use `IStringLocalizer<Resource>`. For every new key, add German, English, and French translations in `EnvelopeGenerator.Application/Resources/Resource.de-DE.resx`, `EnvelopeGenerator.Application/Resources/Resource.en-US.resx`, and `EnvelopeGenerator.Application/Resources/Resource.fr-FR.resx`.
 
 ---
 
@@ -210,3 +211,7 @@ This plan is prepared to remove the DevExpress PDF viewer dependency from `Envel
 5. **Save/Send payload parity verification**
 6. **Session/cache removal cleanup**
 7. **End-to-end testing and UI polish**
+
+### Implementation Progress Notes
+
+- Localization requirement clarified: all new Sender Editor UI strings must go through `IStringLocalizer<Resource>` with DE/EN/FR resource entries.