Alignment languages didn’t make it into late D&D editions, but they do make a little more sense in a religious context. They don’t even have to be languages: perhaps, although it’s written in Common, only the faithful Cuthbertian can quote you chapter and verse from the Chronicles of St. Cuthbert. The unfaithful cannot bring themselves to say the words. (Under these rules, the Devil cannot quote scripture. If he does, he takes Radiant damage.)
Furthermore, people of opposed alignments cannot understand words quoted from a religion’s holy writings. To a priest of Nerull, the scripture of Pelor, even if spoken by a peasant in simple Common, will sound like “argle bargle zip nip” or whatever.
Unaligned/neutral people — people who haven’t chosen a deity to worship — can understand the holy words of all religions. That’s how they’re proselytized.
Thus, common quotes from scriptures can be used to fulfill the original purpose of alignment languages: finding people of the same moral code, without allowing much else in the way of secret communication (unless, Dogs in the Vineyard style, you allow players to make up apropos verses from holy books). That seems to be in keeping with the original intent of alignment languages:
As for alignment languages … it seemed to me that each such groups would have developed their own patoise as a recognition means, more or less like secret societies have signs and signals to ID their fellows.
Never did I envisage characters announcing their moral-ethical (or lack thereof) beliefs and convictions. Rather, the alignment languages were meant to be the means by which one might discover a like-natured individual. Similarly, conveyance of information or general conversation was not contemplated using such “language.”