How do you spell Mariah in french?

If 'Mariah' is pronounced 'MaRYa'(as in 'black maria'), to enable a native Spanish-speaker to read it as such, it would help to write it:

María (with accent on the middle 'a').

Otherwise, the Spanish-speaker may well pronounce 'Mariah' as 'MahREE-ah' (as it's pronounced in the film 'West Side Story', for example).