Something I had learnt a while ago, thought I had taken note of it somewhere but looks like I didn’t.
If SharePoint online starts to show text in a different language, and you get a warning that you can’t set the language as it is managed by the organization, the issue is your browser.
In Edge, for instance, I have the following order:
And SharePoint defaults to English UK for me. If I change this order, or add a new language like Italian on top, SharePoint Online switches to that language.
Another thing is the AD attribute preferredLanguage. If that is set, then the browser settings are ignored. It can be set to, for example, en-US to enforce English US for everyone. This table’s useful I suppose. The attribute needs to sync to Azure AD once its set, before SharePoint Online honours it.