I have observed (in testing) that if I promote my first domain controller using an English version of Windows server and then the second with the French version, the names of many of the items in ADUC and ADSS retain the English names.

Full dtails of my experiment are in this short blog post:

http://davidmtechblog.blogspot.com/2015/08/active-directory-domain-controllers-in.html

For example, even on the French domain controller (DC4), "Account operators" (and all the other groups) retain the original English name.

On the other hand, in a purely French environment, the names of these groups are indeed translated.

Is this behavior typical?

Would the language of the first domain controller always take precedence?

I'm thinking this could be a challenge if, at first, the domain had only Polish domain controllers and then, the company expanding, domain controllers in based on Windows server in another language were added.

Would admins in the US or UK have the names of the groupes referenced above all in Polish?

There is one Builtin container for the domain, so all of the builtin groups, like "Account Operators" and "Administrators", are the same for all domain controllers in the domain. You can rename these groups in ADSI Edit, but the new name will appear in all DC's. I noticed that I don't seem able to rename the groups in ADUC, but I can in ADSI Edit (as a member of Domain Admins).

Edit: You can use Group Policy to configure regional settings if this help: https://technet.microsoft.com/en-us/library/cc754020.as