It's completely understandable to not wish to see chat of other languages, but I find a few problematic things with this suggestion:
1) Separating languages from players via a command (i.e. your suggestion) would be hard to enforce; theoretically, staff would likely be required to "force" the command on players, as not every non-English speaking player would use it.. likely requiring a very redundant rule.
2) Bilingualism. Many players who speak English also speak other languages, which would make it tedious to alternate between filtered & non-filtered chat to be able to contact English speaking players. I even learn some Spanish via SA:MP.
3) Less reports. Non-English speakers may point out a cheating player (words like "cheats" are universal in SA:MP), and if the chat is filtered by someone who can record (or report on Discord), that player won't see it.
Still, I don't want people ignored by me... If they want to tell us something, I can't hear them.
Like my second point addressed, it would be tedious to alternate between toggling chat filter on & off. It's probably best that you can hear them on the off-chance that they need to talk to you or alert players of a cheater