"Esperanto is a cool language!"
Translation:Esperanto estas mojosa lingvo!
Here's a snippet I collected and translated a while back from a forum on the topic:
'Mojosa signifies nothing special apart from 'plaĉega.' For the youth, it is simply a less 'ordinary' synonym. Youth characteristically like nothing which is commonplace, ordinary. Youth are inclined to be bored by many things, among themselves and also by language, and for this reason they endeavor to contrive always some other synonyms....these kind of 'youth-stiled' words are not worthy of being guarded in word stocks, because after few decades it is possible to accumulate even 20 synonyms for the same word. For example, Hungarian has during my lifetime already accumulated 20 words for the word 'person.' Therefore, this tendency is not supportable [apoginda] in a universal, planned language."
"Esperanto mojosas" or "La esperanta lingvo mojosas". Note that the second is a little redundant, like saying "the English language" when "English" is usually enough. "Esperanto mojosas" means a bit more than "Esperanto estas mojosa" ("Esperanto is cool"), it's more active: Esperanto is intrinsically cool, coolness is its nature, it exudes coolness. :-)