I’m sorry, I can’t reproduce the issue. In my tests the customized text shows up on the client even when there’s no existing translation at the time of customization.
That’s on our roadmap. I’m hoping we can get this done within the next 6-12 month.
In that case using “Customize Text” is the right choice. That’s exactly why we built that feature.
The way I usually do it is that whenever I see an untranslated string, I go to Crowdin, search for the untranslated string and contribute my translation. I know that going through the whole list of untranslated strings on Crowdin can be intimidating, especially if you don’t know where those strings are used. That’s why I recommend doing it the other way around.