demux: mkv: use "eng" as the default language.

For each track the language element needs to be present of the default value is "eng" [^1].

[^1]: https://www.rfc-editor.org/rfc/rfc9559#section-5.1.4.1.19

modules/demux/mkv/mkv.cpp

@@ -957,7 +957,7 @@ mkv_track_t::mkv_track_t(enum es_format_category_e es_cat) :
             /* fall through */
         case VIDEO_ES:
         case SPU_ES:
-            fmt.psz_language = strdup("English");
+            fmt.psz_language = strdup("eng");
             break;
         default:
             // no language needed