Well, Japanese doesn't have gendered pronouns you know.
Those are localized into languages that refuse to accept ungendered pronouns (and in some languages, ungendered nouns. French people have opinions on the gender of bridges). From what I can tell, Mordred gets pretty damn pissed off when explicitly referred to as a woman
@Blank Mage dont forget that rather than making enemies into friends with the power of sparkles and rainbows and all the shit most similar stories do, Nanoha makes friends through the application of super-powered beam style ass kicking.
Vivio coming in and wondering about what Nanoha-mama and Fate-mama re doing is just too cute.
You can pretty easily talk about other people without implying gender as well about yourself, but when you do want to imply it, you do have ways to do it as well. And yes, Mordred get pissed when you specifically refer to her as a woman.
I wouldn't say "he" is accurate either, Mordred isn't trans. It's more that she doesn't want to be treated like a woman because she grew up hiding her gender in order to be one of the knights of the round table, as part of her infiltration to kill Arthur. Similar to King Arthur assuming the role of being a "man" solely for the sake of being the King. Mordred wants to be treated like she's on par with the guys in terms of rough housing, combat, and being "manly".
She's also Tsundere and gets mad about a lot of things, I don't think she 100% rejects the notion of being treated like a woman, necessarily, it's mostly that she finds it embarrassing. That's the main thing, I don't think she has a problem with being referred to as "she", Japanese or other language translation, it's just that she doesn't want to be treated like a woman, because her idea of being a knight and a warrior is associated with the dated idea that those are things guys did. Which makes sense because she's from old England.
Ultimately at the end of the day Mordred is a conflicted character, who was thrust into that role by her mother, Morgan. Mordred desires the acknowledgement and parental love of Arthur while also being destined to kill her.