Add a language option to the VR menu, and fix a crash switching to it

- New "Language" row on the Main Menu tab (English/French/German),
  backed by a new "general" group in questshock.config.json
- Fixes a device-reproduced crash: switching language used to call
  language_dealfunc(), whose render_run()/opanel_redraw() repaint
  assumes the game's own in-game options panel is on screen - crashes
  (SIGSEGV) when triggered from our independent VR overlay instead.
  Now calls language_change() directly, which is all that's actually
  needed to reload resources and refresh the inventory/MFD/side-icon UI
- Fixes a second crash: this data set ships frnstrng.res/gerstrng.res
  but not mfdfrn.res/mfdger.res, and language_change() closes the
  current resource files before discovering the new ones don't exist -
  leaving no valid resource file open and crashing on the next lookup.
  Now checks both files exist before ever switching, refusing the
  change (with an in-headset popup) instead of corrupting resource state
- extract_assets.sh can now also pull mfdfrn.res/mfdger.res out of a
  System Shock: Classic installer (which includes them, unlike Enhanced
  Edition) if one is present in res/assets/ - entirely optional
- Documented all of the above in README.md/CONFIG.md

android/app/src/main/cpp/questshock_native.c

@@ -118,6 +118,13 @@ Java_de_ladkau_questshock_MenuOverlay_nativeRequestSetResolution(JNIEnv *env, jc
     qs_config_request_set_resolution(resolutionIndex);
 }
 
+// MenuOverlay's Main Menu tab's Language row - same enqueue-only reasoning as above.
+JNIEXPORT void JNICALL
+Java_de_ladkau_questshock_MenuOverlay_nativeRequestSetLanguage(JNIEnv *env, jclass clazz,
+                                                                jint languageIndex) {
+    qs_config_request_set_language(languageIndex);
+}
+
 // Read-only queries - safe to call directly (no enqueue needed, see
 // qs_config.h's own comment on each of these).
 JNIEXPORT jboolean JNICALL