I have a bias on the subject, my every day communication is in Spanish, but I use English for all technical stuff.
I think that having a mixture of languages, where most people will use English will discourage new participants to post in their native language. Making “redundantly_explicitly” can have the problem that someone may post in a language with minimal participation and may not get a timely response.
I think that if there is a group of people willingly to support a language other than English, it will be a good thing to have a communication channel for that language. It will always have the trade-off of not be the main channel and some important ideas, may not get feed back from that language group.
Life is not perfect, I am just trying to provide the most balanced opinion as a bilingual contributor.