Currently in IB Spanish. Well in Spanish the word Happy translates to Feliz and the word Birthday translates to cumpleaÃ±os. So the correct way to say it is Feliz Cumpleanos. However unlike in the United States, Spanish culture differs by region and most people just congratulate the person by saying something like Felicidades.
The formal meaning of flex is to show off your muscles, and the slang meaning broadens that to just showing off in general, usually with money. Hip hop duo Rae Sremmurd condemned the activity of flexing in No Flex Zone, and once Nicki Minaj jumped on the remix, it was brought into the mainstream.
Slang is colloquial language, where words mean something other than their formal meanings, or where words are used that are not actual English words. (Sometimes slang or vernacular words will become dictionary words through use and custom.) Examples of slang :