"Greek is spoken."
Translation:Τα ελληνικά ομιλούνται.
Languages are plural indeed, unless you actually use the word 'language' (γλώσσα), in which case you need to match its gender and number:
η αγγλική γλώσσα - τα αγγλικά, η γαλλική γλώσσα - τα γαλλικά, η ιταλική γλώσσα - τα ιταλικά, οι ευρωπαϊκές γλώσσες -the european languages.
Or perhaps it's related to the use of the neuter plural form of adjectives being used as adverbs, e.g. μιλάω αγγλικά "I speak Englishly, in an English way" like μιλάω γρήγορα "I speak quickly" from γρήγορος, and that this adverb was then treated as a neuter plural noun since it looks like one.
It's the other way around; nothing is being added; instead, the difference is that the omicron is usually removed :)
ομιλώ is the old verb and this is usually shortened (and conjugation class changed) to μιλάω in general speech, similarly to how ημέρα can become μέρα and ευρίσκω (pronounced εβρίσκω) is βρίσκω nowadays. Also related is αποθνήσκω (aorist απέθανα) turned into πεθαίνω (πέθανα) with a removal of the alpha and a newly-formed present stem based on the aorist stem.
But in the passive voice, ομιλείται/ομιλούνται is more common, in my experience, than μιλιέται/μιλιούνται.