The Content translation module provides a mechanism to flag translated entities as possibly being outdated. The Flag other translations as outdated flag provides a way to make a note of entities that will need updated translations:

This flag does not change any data, but rather provides a moderation tool. This makes it easy for translators to identify content, which has been changed and requires updating. The translation tab for the content entity will highlight all translations, which are still marked as outdated. As they are changed, the editor can uncheck the flag.