website/docs: document language settings (#21968)

website/docs/customize/interfaces/_generalattributes.mdx

@@ -33,4 +33,11 @@ settings:
 
 #### `settings.locale`
 
-The locale which can be configured in the user settings by default. This can be used to preset locales for groups of users, but still let them choose their own preferred locale.
+The locale used by the interface. Set this on the default brand to configure the language instance-wide, or on user and group attributes to configure defaults for specific users or group memberships. Users can still choose their own preferred locale in user settings.
+
+**Example**:
+
+```yaml
+settings:
+    locale: en
+```

website/docs/sys-mgmt/brands/custom-css.mdx

@@ -182,6 +182,8 @@ Think carefully before enforcing a specific color scheme, as it may conflict wit
 
 ### Hide the locale selector
 
+Use this when a brand should use a fixed language and users do not need to select another language on authentication flows.
+
 ```css
 ak-flow-executor::part(locale-select) {
     display: none;

website/docs/sys-mgmt/brands/index.md

@@ -71,6 +71,15 @@ When using the [Mutual TLS Stage](../../add-secure-apps/flows-stages/stages/mtls
 
 Attributes such as locale, theme settings (light/dark mode), and custom attributes can be set to a per-brand default value here. Any custom attributes can be retrieved via [`group_attributes()`](../../users-sources/user/user_ref.mdx#object-properties).
 
+To set the UI language for a brand, add `settings.locale` with a supported locale code. For example, to use English across the default brand:
+
+```yaml
+settings:
+    locale: en
+```
+
+To hide the language selector on authentication flows, see [Hide the locale selector](./custom-css.mdx#hide-the-locale-selector).
+
 ## Image optimization
 
 When you use images and icons for a brand's logo, favicon, etc., be aware of the following optimization tips: