fix hardcoded setfr in main file

README.md

@@ -204,7 +204,7 @@ change the default without having to pass flags every time.
 
 ### 4.3. Locale shortcuts
 
-The `[general]` key `SET_LOCALE` accepts a comma-separated list of
+The `[lang]` key `SET_LOCALE` accepts a comma-separated list of
 `alias:locale` pairs. Each pair generates a function of that name at startup:
 
 ```ini
@@ -214,6 +214,16 @@ SET_LOCALE = fr:fr_FR.UTF-8, us:en_US.UTF-8
 This creates `setfr` and `setus`. Use `setlocale <locale>` to switch to any
 installed locale directly.
 
+Set `DEFAULT_LANG` to one of the defined aliases to activate that locale
+automatically at login:
+
+```ini
+DEFAULT_LANG = fr
+```
+
+If `DEFAULT_LANG` is set but does not match any alias in `SET_LOCALE`, a
+warning is displayed and no locale change is applied.
+
 ### 4.4. Prompt theming
 
 The prompt appearance is controlled by two mechanisms that are applied in order