@IKhitron Thank you for writing this ticket! I have been testing this issue, but I'm not quite sure I understand the bug correctly. So, I have a few follow-up questions to clarify any possible confusion:

1. In step #4, when you write, "Click on the global preferences button," do you mean the "global preferences" link (see screenshot example below)?

2. In Step #5, when you write, "Expected: user interface language 'en,,'" do you mean that the dropdown for Special:GlobalPreferences#mw-prefsection-personal should have your global preference (so, in your case, "en"), but it displays your local preference in the dropdown instead ("he")?

3. In Step #7, are you saying that you changed the global preference language selection in Special:GlobalPreferences#mw-prefsection-personal to "en," since you saw "he" (and it was incorrect, so you wanted to correct it)?

4. In Step #10-11, when you say that you expected to get "en" but you got "he," are you referring to the saved selection in the dropdown in Special:GlobalPreferences#mw-prefsection-personal?

Once we have more information, we can conduct an analysis. We look forward to your response. Thanks!

Hello and thank you for your answer.

1. No, I do not. I'm actually talking about the button "Set your global preferences".
2. Indeed.
3. Indeed.
4. Indeed.

Thank you clarifying, @IKhitron! We have 2 more questions:

1. Do you know if this is longstanding behavior, or does this seem to be something that changed recently?
2. Are you able to successfully select your global and local preference for the correct interface language, or is the end result is that you are never able to complete the action?

Thank you!

Unfortunately, I do not know. I payed attention just before the task creation.
Well, I went to enwiki global preferences, and changed "he" to "en". It still shows he on both sites, but it does work now.