Building on the several useful posts in this thread.
Firstly, changing your in-game language has nothing to do with which server farm you connect to.
It looks like Supercell uses Amazon to host their game servers and Amazon have several data centres around the world for companies to use. Because of the global nature of the game, again, an assumption would be that Supercell pay for global access to all the Amazon locations.
Once you game starts, it will determine which server farm is closest to you PHYSICALLY based on you IP address. You want to have the servers closest to the user respond as they will have the least lag.
Therefore if you want to change the servers you connect to, I would expect you will need to use a VPN to get a different regional IP address.
As others have mentioned, when attacking players normally, you WILL come across bases in languages other than your default. There are plenty of attack/defence logs from script languages (Chinese, Korean, Japanese, Arabic) in my logs, so unless they are switching to English for "much lootz!" it means that the players are spread across the servers.
Perhaps, all changing the language does, is send you into another server with a different list of available players? So the same could be accomplished by restarting the game...
