I am logged in with that same account on this OpeStreetMap Forum.
But when I try to log in with this account at “Questions” section at: help.openstreetmap.org, it simply rejects to do that, as if the account does not exist.
Not sure why is that happens. If need to register a second account there, then that would be a bit confusing again.
I’m not sure why you can’t login to help.openstreetmap.org (you should be able to use your normal OSM credentials). The site admins would be best placed to answer that question, and they’re normally to be found in the #osm-dev channel over on IRC (yes, that’s another place…) - see http://wiki.openstreetmap.org/wiki/IRC for details.
This forum tends to be relatively low volume for English-language development-type queries, but if you want to ask a question in (say) German, Russian or Dutch you’re more likely to find an answer here than elsewhere.
I’ve never been using IRC in my life (I seem to be born too late), and I am not very fond of mailing lists. It’s a terrible way of communication.
In my modest opinion, forums are the best way for any sort of community related content.
I didn’t understand you this part.
If I would like to ask my questions exclusively in English language, then it’s “better” to ask them in here or in “Questions” section at: help.openstreetmap.org?
The FAQ for the Openstreetmap help site may assist in deciding what is better suited to a particular site. FAQ OSM Help and a few points on OpenStreetMap Help too