gnome/yelp: fix `xml:lang` attributes

itstool detects a language code from the mo file’s basename,
so when 26c1869a14
changed the file name to be prefixed with project name,
values like “my-project-xx” ended up in the `xml:lang` attribute
of the generated page files, instead of the expected
IETF BCP 47 language tag.

Let’s fix it by passing a locale code to itstool explicitly.

mesonbuild/modules/gnome.py

@@ -1357,7 +1357,7 @@ class GnomeModule(ExtensionModule):
                 l_subdir,
                 state.subproject,
                 state.environment,
-                [itstool, '-m', os.path.join(l_subdir, gmo_file), '-o', '@OUTDIR@', '@INPUT@'],
+                [itstool, '-m', os.path.join(l_subdir, gmo_file), '--lang', l, '-o', '@OUTDIR@', '@INPUT@'],
                 sources_files,
                 sources,
                 extra_depends=[gmotarget],