I have had some success using regedit.  The most recent time I did this was with Studio 2014 or 2015, when the language codes in the registry appeared to be the same as they were for Trados 2007.  A nice list is available at https://www.vivtek.com/trados/trados_2007_language_codes.html .  The language codes appeared to be the same as for Trados 2007, but might be different now.  I can't remember the name of the registry key that you need to edit (perhaps it's the one Paul Filkin references below), but it will be obvious because you will see a single 10-digit number containing 5 2-hexadecimal-digit language codes which match your current language settings.  Just change this number to match the languages you want, and save.  All the usual precautions for registry editing apply.

I have had some success using regedit.  The most recent time I did this was with Studio 2014 or 2015, when the language codes in the registry appeared to be the same as they were for Trados 2007.  A nice list is available at https://www.vivtek.com/trados/trados_2007_language_codes.html .  The language codes appeared to be the same as for Trados 2007, but might be different now.  I can't remember the name of the registry key that you need to edit (perhaps it's the one Paul Filkin references below), but it will be obvious because you will see a single 10-digit number containing 5 2-hexadecimal-digit language codes which match your current language settings.  Just change this number to match the languages you want, and save.  All the usual precautions for registry editing apply.