Even if you are extremely confident that the artist would have no problem with you doing this without their explicit consent, it's still an act of courtesy and a sign of respect to ask permission first.
Problem with this scenario though, is I'm sure more than one artist worked on those. Also doujinshi's are self published. To make matters more difficult, Chrono Trigger is over 13 years old. This predates common use of the internet by doujinshi publishers, so contact information for the artists may not be available. (Unless these are recent)
On the other hand, doujinshi were never planned for release outside of Japan. Translating it to English has little to no effect on the artist's income for their work.
If you can't find contact information, then that's a different issue. If you can't, it's still an issue of respect to contact the people involved. It's not about their income (although you don't know if an arbitrary artist may be wanting to branch out to the English speaking world, in which case it is) but about courtesy.
I would want to know if someone was translating and rereleasing something I made, even if I had made it 13 years ago. I suspect you would as well.
