It's like the Polish umieć, or Mandarin 会 (hui4); it's something you have to study or learn to be able to do, i.e. swim, surf, ski, speak a language, read, cook etc.
I gave the Polish and Mandarin examples in case you would like to directly translate those specific words and find an equivalent to a language you're already familiar with, which I see there are quite a few!
I'm not quite sure what you're asking about here, but if it's the difference between -a/an for any other language and -o/on for Esperanto, I recall it being mentioned in one of the earlier units' tips sections that Esperanto is unique and always gets treated as a noun in these cases.
Which I guess means distinguishing between the language Esperanto and one who actually hopes just has to be via context (and capitalization?).