In addition to what others already said: The French pronunciation is also accepted in Germany and in the view of many more prestigious. I once received bad reactions for using the one in the meme
It's not [kuzɛ̃], but with ng as in "boing". Also unless we're talking loans or Occitan substrate there's no voiced velar nasal in French at all, in a language full of nasal everything.
Then you may be technically able to speak both English and French, but either or both with terrible accent. French cousin and English boing don't end with the same sound.
I speak both fluently and with proper accent. Maybe I don't understand phonetic notation? Please just explain what Germany is saying to France in the meme?
Please just explain what Germany is saying to France in the meme?
Germany is ending the word "cousin" with the same sound as English "boing". There is no simpler explanation, and don't pretend like it depends on knowing IPA. Understanding depends on knowing how "cousin" and "boing" are pronounced correctly.
If you cannot understand it from this, if you can't hear how those two sounds differ, then I can't help you. Maybe listen to some audio files of those nasals until you can tell the difference.