It's rude to post in a language that the reader/listener doesn't understand without a translation because it's exclusionary. At one of my former jobs, three employees were fired because they were saying really nasty things about a co-worker in a language that the coworker didn't understand. Fortunately for her, the customer at the ticket counter DID understand and complained to a manager.
Now, I do see the other side, which is that anyone can use Google Translate on such posts, so maybe the rule shouldn't be necessary.
One reason I could see for the need for English translations are possibly because the creator(s)/developer(s)/mods/whoever is in charge (along with most people who post on the forums here) probably speak fluent English and that may be their first (and possibly only) language. It would be pretty hard to moderate comments in other languages if one of the above mentioned people didn't know the language. Sometimes Google translate isn't accurate or there are phrases in other languages that could have double meanings and possibly be seen an non-platonic. An English example would be skin to skin. There can be nuances to other languages that certain people may only know and this could be something like that. I doubt this rule was created with any bad intentions to harm others. Also yes I do speak some Spanish (poorly lol) but definitely appreciate the translations and would think writing in Spanish or any other language other than English in this particular website could be seen as having a sort of secret conversation in a public setting.
On the flip side, if it's required for the writer to put in the translation, they could purposely not have them say the same thing. 🤷♀️
Why do people argue so much about the forum rule instead of just typing in spanish & just comply