Enforcing Specific language settings using Group Policy or .ADMX file?
Solution 1:
For Office, I think the Office administrator templates could be helpful.
First, please ensure the "specific language" of Office would be installed on users' PC.
Then to set Display language, Configuration\Policies\Administrative Templates\Microsoft Office 2016\Language Preferences\Display Language
For Editing Languages, go to User Configuration\Policies\Administrative Templates\Microsoft Office 2016\Language Preferences\Editing Languages to specify the language.
More information, please refer to Use Group Policy to enforce language settings.