It's hard to say that it's absolutely wrong, but I agree with the other English speakers in the other thread that it's not very idiomatic, and that there ought to be several other better options. It's certainly misleading as the only translation offered.

The problem is partly that 'conquest' is a rather literary word already, and the collocation with 'self-' is even less common, if theoretically possible. Even in the most abstract, philosophical terms, it would sound better to me to use 'self' as a noun, maybe like 'the conquest of the self,' 'conquering the self,' etc.

And in more normal contexts, I would be inclined just to suggest a phrase with a verb; maybe something like 'overcoming one's own reluctance / (worst) instincts,' 'making a special effort,' 'forcing oneself,' etc.

I'll try to think about this again when I have more time and more access to dictionaries, but maybe others can take it from here.

self-conquest
n. the overcoming of one's worst
characteristics etc.
[Australian Oxford Dictionary]

Found one entry in the Australian Oxford Dictionary. All my other dictionaries don't really list this term.

So far the most powerful human beings have still bowed worshipfully before the saint as the riddle of self-conquest and deliberate final renunciation. Why did they bow? In him—and as it were behind the question mark of his fragile and miserable appearance—they sensed the superior force that sought to test itself in such a conquest, the strength of the will in which they recognized and honored their own strength and delight in domination: