When we say "Planar languages," languages like Ignan, Celestial, Rhebus and Abyssal are the first ones to come to mind, the languages of the Planar Outsiders. But are those really the languages that are spoken by humans or other non-exemplar races?
If I go into Arvandor, are the natives of that divine realm speaking Celestial, or are they speaking Elven? Likewise, if I go into Olympus, are they speaking Celestial, or are they speaking Greek? Do the people in Heliopolis speak Celestial or Formian, or do they speak ... like, Coptic?
I've been giving this a lot of thought, and uh, while it may seem like introducing new languages for each pantheon may just confuse matters, it also seems a little unrealistic to suppose they don't exist, and are human prime languages really any less realistic or any more of a hassle than demihuman languages? I don't know that they are.
Thoughts?
My interpretation is that there are different dialects. Also, I DMed that Axiomatic and Anarchic are languages.
With the Inner Planes, I created special house rules involving creole languages, with the base languages being Ignan, Auran, Aquan, Terran, Vivacian, and Entropian. A character needs to know both languages in order to understand and speak a creole.
For instance, paraelemental languages are creoles between two elemental languages, while the lingua-franca on the Quasielemental Planes is a creole between and elemental and positive-negative language. Even among these groups, there are different dialects depending on region (for instance, the Glacian dialect on the Precipice is closer to Auran than it is to Aquan, while the dialect in the Sea of Frozen Souls is the opposite.) The dialect stuff has no effect on mechanics and is simply a roleplayting and fluff issue.
As for realism, in 1 and 2E, just about every race had its own language, so Comprehend Languages and Tongues were far more essential. 3E simplified the whole thing. For example, in 1 and 2E each giant spoke its own language, same with each type of genie, each type of elemental/paraelemental/quasielemental, and each species of dragon. The only common language was sylvan, which was the language of forest-dwelling fey, centauri, satyroi, and similar creatures. Each outsider race likewise had its own language (for instance, the Yugoloths had their own language, as did hags and hordlings. Baatezu and Tanar'ri had languages unique to their race.) In addition to making Tongues and Comprehend Languages so necessary, it was rather difficult to keep track of all the different languages.