Certainly:
There are two translations, but you only saw one. The entire Star Diaries book has been split into two parts, one translated by Michael Kandel, and the other by a team whose last names I forget. What you've seen was simply the two volumes of the same book (with different stories, of course). The work done by Michael Kandel is superb; the other is less so.
But there does exists another version, done by yours truly, which is unavailable in bookstores for the simple reason that it is technically illegal, not endorsed by Lem (though he apparently did like it somewhat), and will not be published because Faber &Faber is not interested in publishing it.
Cheers, Socrates