According to the GLL ruleset:
If you have a 50/50 split between players from different regions, you can choose which region you want to participate in.
However: Each team can only play in 1 region at the time. All achivements and accomplishments within the region the team have been playing in is locked to that specific region for that specific Season. Any accomplishments from the previous season does not carry over if the team chooses to swap a region between seasons. This means that if your team competed in the Americas region during S2, it needs to continue to do so if you want to keep a spot in the promotionals, for example.