@Aronida
Onomatopoeia is sound written out. The best, and easiest, example is simply an explosion written as "Boom".
For laughter, particularly in the confident female persuasion, a good princess laugh is always a good default.
Which, typically and simply, is
"Ohohoho".
"Fufufn" is a good one as well, for a more down-to-earth character like Nagisa. However, the reader should be able to pick up her confidence through her next sentence and her posture.
Which is why I question the necessary usage of the translator note.
Hey, it's been 8+ months, I can't fully expect you to not have some rust. Or, if you're new, not expect you to understand everything off the top.