In English, we practically never use "thou" or "thee." Heck, not even "ye" - and it costed us our ability for a plural "you". Nevertheless, worrying about formality isn't a huge deal in English, since normal speech is already formal, although we do use words like "sir" and "miss". However, I know that other languages have varying degrees of formality, like "usted" being used with practically everyone except family and children one one end of the spectrum, and "u" being almost insulting if you call some Dutch people that.
Anyways - what's the difference between formal speech and casual chit-chat in the language you speak natively or study?
Interesting prompt!
Shamefully, I am not multi-lingual. I only speak non-regional american english, texan, and tex-mex (colloquially known as shovel Spanish).
I must note, however, that local dialects in the american south do possess a plural "you." It's "y'all," although it certainly cannot be considered formal.