add textdomains to appdata file:

One must compile wesnoth with -DLOCALEDIR=/usr/share/locale, as that's the place
where the system looks for translations.
(One can test it with gettext -s -d wesnoth "poisoned")

This is according to the docs, needs more testing if it works.
Doesn't work with Arch, though Arch doesn't support software centres well.

[ci skip]

(cherry-picked from commit 60f2158)

packaging/wesnoth.appdata.xml

@@ -101,4 +101,31 @@
     <binary>wesnoth</binary>
     <binary>wesnothd</binary>
   </provides>
+  <translation type="gettext">wesnoth</translation>
+  <translation type="gettext">wesnoth-lib</translation>
+  <translation type="gettext">wesnoth-editor</translation>
+  <translation type="gettext">wesnoth-help</translation>
+  <translation type="gettext">wesnoth-ai</translation>
+  <translation type="gettext">wesnoth-units</translation>
+  <translation type="gettext">wesnoth-multiplayer</translation>
+  <translation type="gettext">wesnoth-anl</translation>
+  <translation type="gettext">wesnoth-tutorial</translation>
+  <translation type="gettext">wesnoth-test</translation>
+  <translation type="gettext">wesnoth-aoi</translation>
+  <translation type="gettext">wesnoth-did</translation>
+  <translation type="gettext">wesnoth-dm</translation>
+  <translation type="gettext">wesnoth-dw</translation>
+  <translation type="gettext">wesnoth-ei</translation>
+  <translation type="gettext">wesnoth-httt</translation>
+  <translation type="gettext">wesnoth-l</translation>
+  <translation type="gettext">wesnoth-low</translation>
+  <translation type="gettext">wesnoth-nr</translation>
+  <translation type="gettext">wesnoth-sof</translation>
+  <translation type="gettext">wesnoth-sota</translation>
+  <translation type="gettext">wesnoth-sotbe</translation>
+  <translation type="gettext">wesnoth-tb</translation>
+  <translation type="gettext">wesnoth-thow</translation>
+  <translation type="gettext">wesnoth-trow</translation>
+  <translation type="gettext">wesnoth-tsg</translation>
+  <translation type="gettext">wesnoth-utbs</translation>
 </component>