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 the use of words that are not considered standard English. It should never be used in academic or professional writing. Often, these words are developed from fads or simple laziness. Sometimes slang is used by a given group and those outside the group do not understand it. Slang can also be insulting to some people or groups. Some slang is associated with certain time periods. In the 1960s, young people used terms like groovy, cool, dude, far-out, and trippin'! Some of those terms are still used.