Add language customization flag (#7374)

This allows you to customize any string (that has a translation) or add your own translations.

docs/guide.md

@@ -22,6 +22,9 @@
   - [Proxying to a Svelte app](#proxying-to-a-svelte-app)
   - [Prefixing `/absproxy/<port>` with a path](#prefixing-absproxyport-with-a-path)
   - [Preflight requests](#preflight-requests)
+- [Internationalization and customization](#internationalization-and-customization)
+  - [Available keys and placeholders](#available-keys-and-placeholders)
+  - [Legacy flag](#legacy-flag)
 
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@@ -458,3 +461,45 @@ By default, if you have auth enabled, code-server will authenticate all proxied
 requests including preflight requests. This can cause issues because preflight
 requests do not typically include credentials. To allow all preflight requests
 through the proxy without authentication, use `--skip-auth-preflight`.
+
+## Internationalization and customization
+
+code-server allows you to provide a JSON file to configure certain strings. This can be used for both internationalization and customization.
+
+Create a JSON file with your custom strings:
+
+```json
+{
+  "WELCOME": "Welcome to {{app}}",
+  "LOGIN_TITLE": "{{app}} Access Portal",
+  "LOGIN_BELOW": "Please log in to continue",
+  "PASSWORD_PLACEHOLDER": "Enter Password"
+}
+```
+
+Then reference the file:
+
+```shell
+code-server --i18n /path/to/custom-strings.json
+```
+
+Or this can be done in the config file:
+
+```yaml
+i18n: /path/to/custom-strings.json
+```
+
+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.