Instead of showing the French word dans, BibLaTeX is putting in to introduce booktitles and journals. For example
@incollection{rodgers99,
Address = {London},
Author = {Roders, C.},
Booktitle = {Modern France: Society in transition},
Date-Added = {2013-03-30 21:06:06 +0000},
Date-Modified = {2013-03-30 23:09:27 +0000},
Editor = {Cook, M. and Davie, G.},
Pages = {53-72},
Publisher = {Routledge},
Title = {Gender},
Year = {1999}}
comes out as
Sellier, G. (2002). « Construction des identités de sexe dans les séries policières françaises. » In : Les séries policières. Sous la dir. de G. Sellier et P. Beylot. Paris : L’Harmattan, p. 259–271.
as opposed to
Sellier, G. (2002). « Construction des identités de sexe dans les séries policières françaises. » Dans : Les séries policières. Sous la dir. de G. Sellier et P. Beylot. Paris : L’Harmattan, p. 259–271.
Obviously, I've included babel and everything (hence most of it is translated), so I can only assume this is a bug?
I've only just started using LaTeX, can I just edit the french.lbx file in the biblatex package or is that a bad idea?

french.lbxfrom thebiblatexinstallation of your TeX system. If you manually change a file installed by your system these changes can be overwritten by updates or re-installs. – moewe Aug 06 '20 at 12:14