We are an English site. As such, everyone who posts (questions or answers) needs to write their posts in English.
Even when English is not spoken as a first language, this shouldn't be a problem for anyone who puts in some modicum of effort. If they know enough English to understand the question they're answering, they should know enough English to write the answer in English.
Sure, it might not be the best English, but let's be honest here: Even native English speakers can has attrocious grammer sometimes.
If one is having trouble explaining a particular term or concept in English, if it's essential to the answer then he should by all means include the original term (usually after trying his best to explain it). And in many cases, levels of expertise pretty much necessitate learning foreign jargon (I, for example, am hardly fluent in Arabic, but I'm not going to be lost when someone throws around terms like مرابحة or عربون); foreign terms often lose too much in translation to be useful in any other form.
If, however, someone puts absolutely zero effort into writing their post in English, that to me is no different than someone who puts absolutely zero effort into making their post intelligible, or who puts absolutely zero effort into ensuring that their post even answers the question. Such zero effort posts should in most cases be downvoted as unclear and/or flagged as VLQ, because that's exactly what they are.
And, quite frankly, if one doesn't know enough English to effectively communicate in English, then perhaps an English-language Q&A site isn't the best place to participate.