language: remove iso-codes dependency and XML parsing code

The ICU library will be used instead of the iso-codes package to get the
display names from the language codes
(e.g. "fr_BE" -> "French (Belgium)").

To avoid duplicates g_tree_replace() was used before this commit. It's
now replaced by simple list traversals. The list is small anyway,
performances should really not matter, so simple code is preferable.

https://gitlab.gnome.org/GNOME/gspell/-/issues/14