As the title says, regions such as EUW are only allowed to have English, French etc. after Riot removed the ability to change locale. Some people that have moved from other countries to EUW may not feel as comfortable reading and hearing these languages and would prefer their first language such as Chinese, Korean, Russian etc.
after Riot removed the ability to change locale
You can still change the language by appending " --locale=xx_XX" at the end of your LoL's shortcut target path.
For instance, if you want to change your language to Korean, it should look like this:
"X:\......\League of Legends\LeagueClient.exe" --locale=ko_KR
You can even create a new shortcut with a different locale for the language that you want. When you launch the client using that shortcut it will switch to that locale.
Beware that there's a bug which causes the game to crash if you change your locale to "ja_JP"(Japanese) when someone in either team has a team pass. This bug affects all game modes except TFT.
No reason not to allow it, but such cases are so rare that you're probably the first person ever pointing out this issue. Korean and Japanese immigration is basically non-existent in Europe, Russians are few while young Chinese are almost always born here (and therefore have no language-related problems, save for rare exceptions). I can't think of any other non-Western idiom.
