Take weights into account when matching systemLanguage against the user's preferences
See the part where the spec talks about the
switch element and the implied
allowReorder=yes attribute from SMIL, if there are
systemLanguage attributes in the switch. As far as I understand this, it means that we should take weights into account when considering the user's language preferences. See also #356 about allowing a full
Accept-Languages HTTP header to be passed into librsvg with language tags and weights.
When librsvg uses the environment's language settings (i.e.
LC_MESSAGES on Unix), I'm not 100% sure what the weights should be; maybe just give preference to languages listed first?