If all cultures have the right to self-determination, what of the right of self-determination of a culture that determines it wishes to conquer other cultures? In order to respect its right of self-determination, one would have no choice but to permit its aggression.
No, because that is not self-determination, that is aggression. It is, in other words, the deliberate valuation of one's own culture's power over another culture's and a denial of that other culture its
right to self-determination.
The right to self-determination, more specifically, refers to the right of any culture to make decisions for itself that do not interfere with the rights of other cultures to make decisions for themselves.
To make a symbolic analogy: I have the right to swing my arm wherever I want so long as it does not hit your nose.