When URL Translations are enabled, and when creating a translation of an entry, changing the Title field manually is common practice b/c you’re translating the text, but the JS that creates the URL Title field value based on the Title field value does not trigger on an existing entry, so the URL Title field has to be manually re-typed as a valid slug, or left as the default language value.

Look into adding a button next to the URL Title field, perhaps a simple “refresh” icon, that will re-generate the slug (calling EE’s native JS) based on the Title field value, thus making it easier to add translations.