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Pretty sure Serebii made it clear that in this case, it's Form :<
well, depends on how you translate it, really. Whereas a lot of "formes" have an English name in the Japanese version (transliterated as "forumu"), Keldeo's are in Japanese. They're originally "Futsuu no Sugata" (Normal Form) and "Kakugo no Sugata" (Resolution Form). In English, I'm pretty sure "form" and "forme" are interchangeable (not sure if it's an American/British distinction, or what), so either one of those could work. However, it's worth noting that "sugata" could also translate to any number of synonyms, such as "appearance", "state", etc. At any rate, it's not named like the typical "forme".