Clerics start with a language pertaining to their alignment, rather than their deity. This was supposedly going to be looked at with moving them to domains or allowing them to select one on creation, but I'm unsure if that will ever happen at this point. (It started to, though, with domains like Good and Evil.)
My proposal here is simple if there is no other plans to modify cleric languages: ensure that Clerics stick with the language they're granted on creation. This means that if a good-aligned cleric shifts Neutral, they won't automatically lose Celestial. This is sometimes difficult to explain in roleplay, not to mention that Neutral doesn't get a language. The same for evil with abyssal/infernal.