Magnus,
That is very cool! I also noticed the same trend, which indeed does at first glance appear to be a male bias in the language wikis. However, drilling down into the actual data it's really not so bad. What you actually see is a strong bias from the ULAN for artists who have been active in the US for whatever reason (artworks in US museum, exhibition, or public art). The RKD has a similar strong bias for artists who have been active in the Northern & Southern Netherlands and adjoining countries (nl, fr + de). We should offset this by looking at other databases, and also looking at the same cross-wiki data for all biographies with occupation=artist.
Jane