Nonbinary

In French, the plural they is the masculine ils for mixed gender groups, and only the feminine elles for groups that are exclusively feminine. On is a singular gender neutral form of they often used in situations with unknown gender.

I would use on for singular and ils for plural when talking about a non-binary person because that seems the most reasonable application of the language as I understand it.

However as a non-native speaker I would defer to whatever native speakers do. Can't say I know what the established best practice is.

Spanish is my first language but nowadays I rarely use it, so I just stick to default stuff. If I still activity spoke Spanish everyday id probably make a effort to change up the grammar.

I generally prefer neutral language (person, they/them, etc.), but I go through phases where I don't care if I get called he or she instead.