It is indeed not correct usage of the word "I'll", English is weirdly inconsistent as always.
"I'll" cannot be a complete sentence.
I wish English did work that way but it doesn't.
I'll is a contraction meaning "I will" thus it can be a complete sentence
Strike thy balls, I'll.
/j
Nope.
It's.
It is a contraction of "I will", meaning that used in this context it actually is grammatically correct, as I am saying "I will watch my language".
This, too, is grammatically incorrect.
Isn't.
This one actually isn't correct
Yesn't
None of them are
They're.
That is also grammatically incorrect.
It's.
Actually, it's.
Perchance.