If I’m not mistaken, there are two types of customizable text content elements: those that will always be displayed in the site’s language and those that will be displayed depending on the respective user’s locale.
If that is correct, I’d like to suggest that this is made transparent for every text element shown under Customize - > Text Content (In other words: add some kind of marker indicating something like “depends on user locale” or “depends on site locale”).
Reason: if you allow users to select their own locale and you adjust some text elements, it’s important to know whether these changes will be visible to all users or just to those using the site’s default locale.
Do you have an example of something that isn’t translatable? AFAIK all Core text is translatable.
I think what you may be experiencing is when a translation is missing Discourse falls back to the English text.
Maybe I’m still not understanding how locales/languages work in discourse, but it was my understanding that when a user selects a different locale (say: French) than the site’s default locale (say: English), this does not entail that discourse will now only speak French with that user. For example, the discobot tutorial will still be in English, right? I don’t know, but I could imagine that also things like password reset emails and sumary emails will remain in English so that basically only the web UI changes language.
If this is so as a rule of thumb, then it’s still not evident from each Text Content element whether it affects the web UI or emails or what.
Why would you want to know? Because it is one thing to customize text elements in the site’s default language (which is what you do under Admin -> Customize -> Text Content) and another to then translate those new texts into the various user locales. The former is a piece of cake and the latter I have not quite figured out yet and even when you know how to do it, you still need to do it.
So, long story short: I would like to avoid customizing text elements which users with a different locale will not see unless I translate them. But in order to do that, I need to understand which text elements are governed by the site locale and which listen to the user locale.