We took the transmediterranea ferry last summer from Barcelona to Ibiza in August (which is substantially busier than June), did everything last minute and had run into some sailings sold out for student rates (and in general). We also had the same problem with language -- my Spanish is practically useless unless you need me to ask where the toilets are or where the beach is.
BUT, in lieu of a) learning Spanish on the fly and b) getting someone else to do the booking for you...
can 1) ask when someone is working who speaks English -- completely lazy/pathetic, I know, but they told us when someone was available and we called back then.
2) You also might try this link
http://www.trasmediterranea.es/deleagen0i.htm & click on agents abroad they might help.
My BIGGEST recommendation re the ferry -- book yourself on the one with the pool -- I don't think it costs any extra and it's SOOOO worth it. I don't even mean for
using the pool -- it's just that the other ferry is wicked slow and, in my opinion, crap. No reclining seats + loooong overnight haul + room full of chain smokers while you're trying to sleep + prelude to holiday with little rest = rough start.
Just my two cents.
Good luck!