I have gotten a few bugs recently which were only discoverable when the system language was set to non-English. We need some way to mock this so that our tests can run in these other environments.
There is internals.overrideUserPreferredLanguages(). Unfortunately, WebKit and WebCore code is not very consistent, and we often check the language and locale directly.
Also, it's not always obvious where we should be using system language, and where we should be using system locale.
See also: https://bugs.webkit.org/show_bug.cgi?id=76138