> The browser vendors don't make it easy to change the browser language

In Firefox it's in the first page of the settings: Tools > Options > Language and appearance, there you can change the language of the UI and the preferred language for displaying pages.

I have my Firefox "Language and Appearance" language set to "English (United States)". I'm not sure why this setting even exists, because the operating system also has a language setting, which I've also set to "English (US)". Yet somehow, some parts of my Firefox UI are still in Japanese. Nobody has ever been able to explain why, or how to fix it.

I don't understand why a web browser (or any application) needs a custom language setting, much less a prominent one. It's just one more place that can fail.

> I don't understand why a web browser (or any application) needs a custom language setting, much less a prominent one. It's just one more place that can fail.

For entertainment including games I want them to be in the original languaugage they were developed in (if it's one I can speak fluently) even if that differs from my system language (which differs from my native language (which at many points in my life has differed from the default language for my location)).

I don't personally have a reason to have a different language for my browser UI though, but perhaps someone else does. However I agree that the default for application languages should be "use the system default" - not whatever was the system default at installation time or some other fixed language.