Add Sepedi language support for song generation

Request: Add Sepedi (Sesotho sa Leboa / Northern Sotho) as a supported language for lyric and song generation. Sepedi is one of South Africa's official languages, so this would expand accessibility for South African users who don't speak Zulu. Impact: Improves language coverage for the South African market and reduces friction for users who can't work in the currently available South African language options.