I'm not saying this to bash open source translations (let alone translators) or anything. A good translation takes a lot of work. That's just how things seemed to be last time I tried, and I don't really have the energy to contribute myself nowadays.
Of course there are other reasons for not using a localised desktop especially if you're a technical person, such as better web searchability in case of problems. But the inconsistent quality of translations is probably one of the top reasons for me.
The translators also likely did not understand which words were key app terms which should stay constant. For example "benefit" is a key term which can not ever be substituted out with something like "improvement" even if it seems like it makes more sense in the particular string. But without knowing the app well, you wouldn't know this.
Seems like it would be better just convincing everyone to use the English versions of everything since those are perfect and the majority of the world knows it now.
There are lots of e.g. elderly and less educated people in non-English speaking countries who don't speak or understand much English, at least not enough to be comfortable. You don't even have to go further than Europe for that. The aging might not be a key target audience for many apps and it might make business sense to not bother with translation, but there's a significant number of people who might be excluded by that, and it would be difficult to accept that as a solution at least for the most commonly needed apps.
It also seems a little presumptuous that English should be pushed as a general solution to everyone in the first place, but you don't even have to go there before you get into practical problems.
As there isn't an easy way to set this per app, it makes more sense to me to just switch the phone entirely to English.