Congratulations for you Khris.
You are one of the few members of this forum (I 'm talking to you as a member, not as (Spanish) mod) who are really care about the forum, how to keep it alive and to increase the action here.
Community groups it's a good idea with a bad execution.
Sorry to say but your post, although it's a good reminder gonna be lost after few days and with no response from any one else.
First of all, groups are almost invisible to any new member of the forum.
I found the Greek group by accident when I was checking Spyros profile and I saw the Greek flag of the group and join there.
When I joined, it was about 15 members (and most of them not Greeks - lol).
Sometimes when I see a new member from Greece (I can recognize many of them by their names), I send him a friendship and a pm saying him to visit the Greek group.
Now it has 35 members.
I have started there some tips (in Greek language).
One problem with the group posts is that there is a limit in the length - the words of the post.
To make it work, I think the idea is to have some section-thread-post, in the upper side of the 1st page of the forum, with a "welcome" in the languages that exist (in the groups).
Something like this.
The post below, was started by me, in off topic section, with the purpose to attract Greek, new members of the forum

but it closed due to unknown language

I don't believe that there must be a full forum (with guides/formations/ ...) for every language but only one general thread.
I understand of course that must be someone like a mini moderator for controlling some bad language or behavior in the threads there
but I believe it's worthy because I can see many people from my country join the forum but cannot follow it because they don't have fluency in English.