In the core rulebook, Tongues is not listed as reversible in the spell tables for Clerics and Blade Dancers. However, it is revisible (the reverse being Garble). I see this as a simple misprint.
However, in the Player's Companion some reversible spells are not listed as reversible for certain classes. For example, Priestesses cannot reverse healing spells or light spells. At least this is my conclusion, correct me if I am in error.
Where I am a little uncertain is the according to the Player's Companion, Witches are the only class that can cast the reverse of Tongues. Is this correct, or was the misprint for Blade Dancers and Clerics carried over into the Player's Companion. I understand that the spell tables were purposely changed in the Player's Companion (the Blade Dancers lost the ability to cast the Command Word spell. So it is quite possible that while there was a misprint in the ACKS base book spell tables for Tongues not being reversible, that the Player's Companion listing of Tongues only being reversible for the Witch was by design and not an oversight.
Here's the original ACKS Rule: Any divine caster can cast both versions of any spell,subject to alignment limitations (e.g. chaotics can freely use finger of death but can only restore life and limb for other chaotics; lawfuls can freely use restore life and limb but can only use finger of death to advance lawful causes against chaos.)
Here's the PC Rule: Some casters can only use one or the other versions of certain reversible spells. Only Witches can cast the reverse of Garble.
Unfortunately the PC version of the rule introduces a flaw in the system because it raises the question of how to count those spells for purposes of number of spells in the repertoire. Do reversible spells count as two spells? If they don't count as two spells, then that punishes a caster that can only cast one of the two versions. If they do count as two spells, then the spell lists are all wrong.
Therefore here's a final revised rule:
Any divine caster can cast any version of a reversible spell on their spell list. They can freely cast the listed version (e.g. Restore Life and Limb or Tongues). If the spell is listed as reversible for them, then they can cast the reversed version when necessary to advance their alignment. If it's not listed as reversible on their spell list, then they can still cast the reversed version, but doing so is a breach of their class code of behavior and they will have to atone.
So the restriction becomes one of code of behavior rather than repertoire.