@nemobis Thanks for thinking about this! I would not want the Finnish interface language for everything because some translations are really bad and it makes searching for strings of text in the UI harder.
The simplest solution would be (as @ninafelwitch pointed out) for Google to just stop forcing their translations on the user. This should be a simple opt-out, there's no need for anything fancy.
However, if they wanted to be clever (and I doubt they do, being clever is a cost centre), then Preferred Languages is an iOS concept, with a well-understood API (see e.g. https://stackoverflow.com/a/30293909) - so it is possible and indeed natively implemented to have multiple 'preferred' languages. I have my preferred languages set to English, Finnish, French, Norwegian and Swedish. Google's apps could parse that...