My host told me to use .htaccess files inside the sub domain folders.
It's too late then. Requests have to be intercepted in the main folder. They won't get any further on their own.
If your host doesn't allow direct mapping of subdomains, you'll have to do everything with rewrites. Which means you have to be
very very careful because the request only gets one chance to arrive at the right place.
You may also want to start shopping for a new host. There's a difference between a small finite number of subdomains-- what you've got-- and wild-card subdomains-- which is what you're basically coding for. It is in the host's best interest to deal with subdomains themselves, because it leaves less room for user error.
Now then...
If you get rid of the htaccess, do all requests end up at your main index page? I'm asking only about the page content, not what the address bar says. That is:
example.com >> user sees the content of example.com/index.html
www.example.com >> user sees the content of example.com/index.html
www.example.com:80 >> user sees the content of example.com/index.html
es.example.com >> user sees the content of example.com/index.html
hu.example.com >> user sees the content of example.com/index.html
fr.example.com >> user sees the content of example.com/index.html
That should be your starting point. I have deliberately included requests that should not end up anywhere, because you will need to deal with those too.