Two opposing votes seem to indicate that the reporter might be wrong. I do not trust my English skills enough to judge this.

Opinions please :-) Factual ones preferred ;-)

For ongoing characters I think djbrianuk is correct. As the character's vanquishing (or not vanquishing) of monsters is something that's continuing in the present, use of the present perfect (have not managed) is appropriate. Personally, I would omit the "yet" from his proposed statement, but it seems correct either way. The message when you have killed monsters actually has the same issue: for ongoing characters it should be "[N] monster(s) have/has perished under your attacks" rather than "[N] monster(s) perished under your attacks".

Note that the current messages actually are correct for dead/victorious characters, as they are finished with killing monsters.

You used past tense, which is correct if the question is, "Did I kill anything?" it works fine. Present perfect tense is correct for ongoing action as Grond says, and may be more correct. Either way works, and works for me and I like the international feel. "Perfect" English is not necessary or desireable in Ancardia I think.

I still think I'm right, but this a pretty minor thing so I'm not fussed as to which way it goes. Technically following my criteria, the actual kill list once you've killed something is also incorrect, viz:

2 monsters perished under your attacks.

The following monsters were vanquished:
2 goblins

Should be

2 monsters have perished under your attacks (so far).

The following monsters have been vanquished:
2 goblins

But as I say this is very minor, hence me ranking it as lowest priority.