It is still a lot more complicated to develop and maintain an application if it uses a different language than english.
Appstore modules rarely come with any translations and when upgrading those, any translations that are done are lost.
When working with a single language it is not even possible to properly reimport those translations (without some extra steps), since this can only be done as part of batch translation, when the “source” language is added to the project.
Even then, this does not always fit: the translation process uses some text matching. If you use different translations for the same source in different contexts, it is not possible to reimport this at all.
There’s other things, like default button titles, that need to be translated every time they are added.
Some suggestions to solve at least parts of these problems: