gtk/language-names.c uses deprecated /usr/share/xml/iso-codes
gtk/language-names.c
parses iso_639.xml
to get language names, but the XML files in iso-codes are considered deprecated by their maintainer since 2016: https://salsa.debian.org/iso-codes-team/iso-codes/-/commit/29d7c457702223ed2724327fa37b1918d855a86b. The JSON equivalents in /usr/share/iso-codes/json
are apparently the preferred form now.
Both are shipped in the iso-codes
package (at least in Debian), but the maintainer of iso-codes has tried to remove the XML version in the past. See also https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=907466.
Unfortunately, parsing the JSON version will require a dependency on json-glib or something similar.