이정도면 한국어 지이인짜 잘하는거잖아요 == 이 정도면 한국어 진짜 잘하는 것이잖아요?
=> Speaking like this is really good Korean, isn't it?
=> To speak as good as this, you must be really good at Korean.
* 이 정도면 (이 정도이면) is a conditional form of 이 정도이다 = is(이다) at this(이) level(정도).
It means "if you're at this level (that I'm speaking)" or "if you can do/speak it as good as this".
* 진짜 잘하는 것이잖아요? is saying 진짜 잘하는 것이에요 emphatically by asking the negative of it rhetorically.
진짜(really) 잘하는(doing/speaking well) 것(thing, that)이에요(it is) means "it is [I am] speaking Korean really well".
멋지당 (멋지다) means "It's [or he/she is] cool/awesome/fantastic", depending on the context.