Throughout The Fifth Element there are several mentions of the guardian race known as the Mondoshawans.
Or rather, not known as the Mondoshawans. Written as the Mondoshawans in all the different sources that talk about them. Because every time their name is said, it is quite clearly pronounced /mɒndoʊˡtʃiːwən/ (even by Leeloo when she’s speaking in the Divine Language)—that is, something you would expect to be represented in English orthography as Mondocheewan or something like that.
All instances of written English on Earth in the movie seem to follow normal, 20th-century orthography, so a priori you'd expect this to be the same.
The name is clearly a loan word in English, borrowed from the Divine Language, and as such you would expect it to be adapted to English orthography, or alternatively to follow orthographic quirks of the source language.
It is of course entirely possible that Mondocheewan represents a dialectal sound change in the Divine Language as spoken by the Mondoshawans (/ʃa/ > /tʃiː/) which was not reflected in the original orthography. But there is no hint of any such thing within the movie, and it seems rather far-fetched to have to go to such lengths of conjecture for a word that happens to be mentioned sort of in passing a few times in the movie.
So I am assuming here that the reason for the mismatch between spelling and pronunciation lies outside the movie itself, out-of-universe. That doesn't help me much, though, ’cause I can't find any indications, interviews, or other hints as to why the writers (or whoever it was) would choose such an unintuitive spelling out of universe either.