The 3rd is supposedly the shortest game in the series so far, if the character counts we have are to be believed. To me, it seems very unlikely that a Zero translation done from scratch can finish before CS3's script is available to them, even if they started right after the 3rd. They'd need more than the one translator they've been using since CS.
I certainly don't think they are
that committed. The series' current translator/editor team is probably going to be the one they stick with, if they can't fit in Zero and Ao anywhere they simply won't get released due to lack of priority. The SC+CS situation of two games being worked on at once probably won't happen again, since that only occurred due to SC taking a lot longer than intended. Basically the entire localization part of project ended up being contracted out, CF finished the translation and half the editing, and Jess who had left XSEED finished the other half.
Which, for what little it's worth, is what I expect to be the most likely outcome.
Ys VIII PS4 was announced for localisation before the JP release though.
The general rule seems to have been that they wouldn't let anyone touch their stuff until it came out in Japan. Since the Vita version was released they were able to start on that one, and because the PS4 version was announced and NISA had every intention of releasing it too, they announced that as well.
Unless Falcom changed their stance, XSEED probably won't get a peek at CS3's script until the Japanese release.
And if I'm not mistaken even Aksys was completely blindsided by the EX+ announcement of Tokyo Xanadu, despite having had just recently announced the Vita version.
Think it's a safe assumption to say they're working on at least Zero or Sen 3 atm, considering Hatsuu said she's working on a big project they couldn't announce this year. I wouldn't be surprised if she was almost solely on Trails now.
Her Twitter account also has a ???? in her project list separate from the Trails series. Could be there to throw people off I guess.