fix(lightdm): inherit LANGUAGE from SMF

--- a/src/session-child.c
+++ b/src/session-child.c
@@ -415,6 +415,7 @@
                 "LC_MESSAGES",
                 "LC_ALL",
                 "LANG",
+                "LANGUAGE",
                 NULL
             };
             for (int i = 0; locale_var_names[i] != NULL; i++)