My first language is not english but I work in a company where everyone is british and white. When I take notes, my supervisor talks suddenly more quickly. In office, colleagues talk with their wired slang. they also use slang with me and then make jokes because I don't answer to what they say ( I genuinely don't understand slang and they make fun of it). When they give instructions, suddenly their english becomes Shakespearean... I am not sure on how to approach it as it is clearly up to their behaviour but it would also affect me.
My boss knows it, but he does nothing. Once I asked not to use slang in office or to repeat some key info. It didn't last long...it caused rumours in the office and didn't last long.
It's frustrating and affecting my happiness in the workplace.