docs: consolidate i18n documentation in guide.md

Replace login-specific customization.md with generic internationalization section in guide.md. This approach is more maintainable, reduces file sprawl, points to source files as truth, and encourages community contributions. Removes repetitive examples that could get out of sync.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: strickvl

docs/customization.md

@@ -461,4 +461,35 @@ through the proxy without authentication, use `--skip-auth-preflight`.
 
 ## Internationalization and customization
 
-You can customize some of code-server's strings for either internationalization or customization purposes. See our [customization guide](./customization.md).
+code-server allows you to provide a language file or JSON to configure certain strings. This can be used for both internationalization and customization.
+
+For example:
+
+```shell
+code-server --i18n /custom-strings.json
+code-server --i18n '{"WELCOME": "{{app}} ログイン"}'
+```
+
+Or this can be done in the config file:
+
+```yaml
+i18n: |
+  {
+    "WELCOME": "Welcome to the {{app}} Development Portal"
+  }
+```
+
+You can combine this with the `--locale` flag to configure language support for both code-server and VS Code in cases where code-server has no support but VS Code does. If you are using this for internationalization, please consider sending us a pull request to contribute it to `src/node/i18n/locales`.
+
+### Available keys and placeholders
+
+Refer to [../src/node/i18n/locales/en.json](../src/node/i18n/locales/en.json) for a full list of the available keys for translations. Note that the only placeholders supported for each key are the ones used in the default string.
+
+The `--app-name` flag controls the `{{app}}` placeholder in templates. If you want to change the name, you can either:
+
+1. Set `--app-name` (potentially alongside `--i18n`)
+2. Use `--i18n` and hardcode the name in your strings
+
+### Legacy flag
+
+The `--welcome-text` flag is now deprecated. Use the `WELCOME` key instead.