This word points at one small corner of England, not at the north in general. It is always written with a capital G, because it is tied to a place. It is not an insult: people from there use it about themselves, and often with pride. You will meet it most in British television, sport and music, where someone's home town matters. The same word also names the way people there speak, so the sentence around it tells you whether a person or a way of speaking is meant.

This use is uncountable: you speak Geordie, the way you speak French, and you do not say 'a Geordie' or 'Geordies' about the speech. It covers the accent and also a set of local words that outsiders often do not know. British television sometimes puts subtitles under strong Geordie speech, which tells you how far it sits from the English taught in classrooms. Saying someone speaks Geordie describes where their English comes from; it says nothing about how good their English is.