Using your native language is no doubt fine if the only people who are going to read your score are speakers of that language. Italian became adopted as the international language of musical instructions so that singers/musicians from any country would only have to learn this limited vocabulary to be able to read scores written by composers from any linguistic background.

Of course, if musical notation were being invented nowadays, the international language of choice would probably be English, but we've rather missed the opportunity for that. I suppose we should be gracious about this - English is already the international language of computers, of tourism, of football, of pop songs, etc etc, so we shouldn't begrudge Italian its dominion in musical notation.

Marje