i18n: add Basque (eu) translation (#12544)

Same as with icelandic (#12301) we may be even fewer than them but let's
have this translated into Basque.

I also volunteer for the basque translation team.

src/os/i18n_locales.zig

@@ -58,4 +58,5 @@ pub const locales = [_][:0]const u8{
     "vi",
     "kk",
     "be",
+    "eu",
 };