Well it does crash if I remove UK files, and defaults to UK on the first launch.
Hmm, I tried removing it, no crash, see attachment.
Also, it doesn't default to en-GB... it defaults to empty string (unless your locale settings of course are en-GB). It just shows en-GB in the combobox, because there is no empty option in the combobox and en-GB is the first in the list (in piratez distro).
When I download newest piratez version, extract, remove all 3 en-GB files, run openxcom, switch master to piratez... after restart the language is set to en-US (the only possible option remaining and pre-selected).
EDIT: all that said, Solar's solution is better (more robust) than deleting the files...