How can I find out which translations the API uses?

Question

Internationalization and localization

There is an application and an api to it, all other services that pull its api pass the localization parameter which locale.
I faced the question, wat a couple of thousand translations, of which I'm sure ~ 20% are not used, how to find out which ones?
If you just clean up all the translations, then 100% something will fall or be missing on the UI or where else the api is used.