Limit locales to 16 common languages, document how to add more

Reduces locale generation from 200+ to 16 targeted locales (major world
languages + Nordic + key European). Saves build time and image size.
Users can add more at runtime via locale-gen.

DOCKER_HUB.md

@@ -125,7 +125,9 @@ The container defaults to English (`en_US.UTF-8`) and neovim as the editor. Over
 | `LC_ALL` | Override all locale settings | `en_US.UTF-8` |
 | `EDITOR` | Default text editor | `nvim` |
 
-All common UTF-8 locales are pre-generated in the image. Example for Swedish:
+Pre-generated locales: `en_US`, `en_GB`, `sv_SE`, `da_DK`, `nb_NO`, `fi_FI`, `de_DE`, `fr_FR`, `es_ES`, `it_IT`, `pt_BR`, `nl_NL`, `pl_PL`, `ja_JP`, `ko_KR`, `zh_CN` (all UTF-8).
+
+Example for Swedish:
 
 ```bash
 LANG=sv_SE.UTF-8
@@ -133,6 +135,15 @@ LANGUAGE=sv_SE:sv
 LC_ALL=sv_SE.UTF-8
 ```
 
+To add a locale not in the list, run inside the container:
+
+```bash
+sudo sed -i '/xx_XX.UTF-8/s/^# //g' /etc/locale.gen
+sudo locale-gen
+```
+
+Replace `xx_XX` with the desired locale (e.g. `ru_RU`, `tr_TR`). This change does not persist across container restarts — for permanent additions, build from source and modify the Dockerfile.
+
 ## Initial Setup
 
 ### 1. Create host directories