Well, in my mother tongue, Kurdish, the case is like English. Mainly word choice and, sometimes, different accents determine your level of formality and informality.
I can speak Farsi too; it's different. In Farsi, they have two forms of language: written and spoken. You may say the same thing with the same words in the two different forms:
Written: Mikhaaham beravam
Spoken: Mikhaam beram
If you use written form when speaking, it sounds old and dramatic! It's like Br.E. in American films, when they want to show a character from the old Gothic ages.
By the way, how's your feeling about Br.E. and Am.E.? Please reply in 'British or American?'