The context of this particular thread, though, is about why the user interface in an app might not be translated well. I responded by way of example explaining

The context of this particular thread, though, is about why the user interface in an app might not be translated well. I responded by way of example explaining how translations can fall behind in a very similar situation, that of a website. Neither of these are the same as the YouTube situation, because in the YouTube situation you're dealing with user-contributed content being translated, which is a crucial difference. Community contributions were never about translating the YouTube UI (or god forbid privacy policy, ToS, etc.); they were only ever about users submitting translations of other users' content.