Make reasonable assumptions or provide good defaults.
Make them overrideable.
Make user settings stick.
I have the same problem, by the way — my phone is in English, which means I get to enjoy Apples hilarious English pronunciation of German street names while navigating.
Same here with Dutch: you could pretty much watch i18n degrade real time by observing the quality of the dutch translations for all major sites and tech products over the years.
Back in the 2000s, dutch wasn't very common but it usually was pretty good (my understanding is that one of the things that helped is that everyone followed the Microsoft style guide for dutch translations?)
Nowadays you get overly literal translations (meaning some form of MTL), translations that don't care for the length of the text (so it gets cut off with ... at the end for interface buttons) and so on and so forth. It all just reeks of automatic translation with little care put into the presentation. This is pretty much a universal experience across every single system I've ever used and why I usually just set all my devices to English. - It's simply not worth it to deal with the botched translations to try and figure out what was actually meant.
I've had the unfortunate mishap of having the perfectly fine English translation of a German site switch to auto translate Dutch crap due to one part using the language preference and the other the IP source, bit weird since as far as I know they usually only offer English. And I can perfectly fine read German so I usually immediately switch to that since some info is not available in English especially with the different Bundesambt websites.
What I would like is a browser and os which allows you to set which languages (multiple) never need translating and the site sticking to that.
Yeah, I think apps just should have a sorted list (/tiers) of languages they support, and pick whatever first has a match in the user preferences. If the app supports two languages equally well, then choose the one first in the user's preference list.
So for me having no,en (in the future where this works I would dare to have no first):
app is in english, has auto translated norwegian: choose english
app is in norwegian, has auto translated english: choose norwegian
app is in norwegian and english equally: choose norwegian
app is in french, has english translation: choose english
People figured this out when they specified HTTP 1.1 in 1997. A prioritized list of the languages the user knows. They even allow a q-factor weighing so the user can specify a factor of how comfortable they are with a language. All modern browsers already support this, but 99% of websites fail to use it properly including Google.
It's like modern apps have forgotten all lessons learned about internationalization.
Same in Windows. The "modern" system apps use just one setting (Windows language) for internationalization, ignoring the old date format and time format settings. So if I set my Windows language to English, I get AM/PM and dot as decimal separator in some parts of the UI and not in others.
Google is notorious for ignoring browser language preferences not only in Youtube but also in its main product, and inferring from the (often faulty) geolocation.
Make reasonable assumptions or provide good defaults. Make them overrideable. Make user settings stick.
I have the same problem, by the way — my phone is in English, which means I get to enjoy Apples hilarious English pronunciation of German street names while navigating.