docs: Added clarification for testing i18n changes (#2612)

Co-authored-by: Marensius Bae Pettersen <marensius.pettersen@soprasteria.com>

packages/admin-ui/README.md

@@ -46,4 +46,5 @@ This report data is also saved to the [i18n-coverage.json](./i18n-coverage.json)
 
 To add support for a new language, create a new empty json file (`{}`) in the `i18n-messages` directory named `<languageCode>.json`, where `languageCode` is one of the supported codes as given in the [LanguageCode enum type](../core/src/api/schema/common/language-code.graphql), then run `yarn extract-translations`
 
+To verify localization changes add `<languageCode>.json` to `./src/lib/static/vendure-ui-config.json` in the array `availableLanguages`. This will make the localization available in Admin UI development mode using `yarn start`