l10n: update of the doc

docs/src/l10n.md

@@ -2,6 +2,15 @@
 
 This guide explains how localization (L10n) is implemented in the **Rust-based coreutils project**, detailing the use of [Fluent](https://projectfluent.org/) files, runtime behavior, and developer integration.
 
+## 🏗️ Architecture Overview
+
+**English locale files are embedded directly in the binary**, ensuring that English always works regardless of how the software is installed. Other language locale files are loaded from the filesystem at runtime.
+
+### Source Repository Structure
+
+- **Main repository**: Contains English (`en-US.ftl`) locale files embedded in binaries
+- **Translation repository**: [uutils/coreutils-l10n](https://github.com/uutils/coreutils-l10n) contains all other language translations
+
 ---
 
 ## 📁 Fluent File Layout
@@ -15,8 +24,8 @@ Each utility has its own set of translation files under:
 Examples:
 
 ```
-    src/uu/ls/locales/en-US.ftl
-    src/uu/ls/locales/fr-FR.ftl
+    src/uu/ls/locales/en-US.ftl    # Embedded in binary
+    src/uu/ls/locales/fr-FR.ftl    # Loaded from filesystem
 ```
 
 These files follow Fluent syntax and contain localized message patterns.
@@ -31,12 +40,11 @@ Localization must be explicitly initialized at runtime using:
     setup_localization(path)
 ```
 
-
 This is typically done:
 - In `src/bin/coreutils.rs` for **multi-call binaries**
 - In `src/uucore/src/lib.rs` for **single-call utilities**
 
-The string parameter determines the lookup path for Fluent files.
+The string parameter determines the lookup path for Fluent files. **English always works** because it's embedded, but other languages need their `.ftl` files to be available at runtime.
 
 ---
 
@@ -155,9 +163,13 @@ In release mode, **paths are resolved relative to the executable**:
 
 ```
     <executable_dir>/locales/<utility>/
+    <prefix>/share/locales/<utility>/
+    ~/.local/share/coreutils/locales/<utility>/
+    ~/.cargo/share/coreutils/locales/<utility>/
+    /usr/share/coreutils/locales/<utility>/
 ```
 
-If both fallback paths fail, an error is returned during `setup_localization()`.
+If external locale files aren't found, the system falls back to embedded English locales.
 
 ---
 
@@ -184,3 +196,15 @@ Fluent default (disabled here):
 ```
     "\u{2068}Alice\u{2069}"
 ```
+
+---
+
+## 🔧 Embedded English Locales
+
+English locale files are always embedded directly in the binary during the build process. This ensures that:
+
+- **English always works** regardless of installation method (e.g., `cargo install`)
+- **No runtime dependency** on external `.ftl` files for English
+- **Fallback behavior** when other language files are missing
+
+The embedded English locales are generated at build time and included in the binary, providing a reliable fallback while still supporting full localization for other languages when their `.ftl` files are available.