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/qs_config.h

@@ -49,13 +49,20 @@ void qs_config_request_load(void);
 void qs_config_request_set_renderer(bool opengl);
 void qs_config_request_set_resolution(int resolution_index);
 
-// Returns true and handles the event if it's one of the four custom events
-// qs_config_request_save()/_load()/_set_renderer()/_set_resolution()
-// enqueue - called from pump_events() (sdl_events.c, see
+// Same cross-thread-dispatch reasoning as the two above - called from the
+// Main Menu tab's Language row (MenuOverlay.java). language_index is 0
+// English/1 French/2 German (gShockPrefs.goLanguage's own numbering,
+// Prefs.h). Applies live immediately, the same way the game's own options
+// menu applies a language change (see qs_config.c's qs_apply_language()).
+void qs_config_request_set_language(int language_index);
+
+// Returns true and handles the event if it's one of the five custom events
+// qs_config_request_save()/_load()/_set_renderer()/_set_resolution()/
+// _set_language() enqueue - called from pump_events() (sdl_events.c, see
 // android/engine-patches/21-android-config.patch) on the engine's own
 // thread, once per polled event, before its normal switch on well-known
 // SDL_* event types. Returns false (no-op) for any other event type,
-// including before the four custom types have even been registered.
+// including before the five custom types have even been registered.
 bool qs_config_handle_event(const SDL_Event *event);
 
 // Read-only queries for the Video Options tab's own selection highlight
@@ -73,9 +80,12 @@ int qs_config_current_resolution_index(void);
 // gShockPrefs.goLanguage (0 English/1 French/2 German, Prefs.h) - read by
 // MenuOverlay.java's/KeyboardOverlay.java's periodic header-refresh timer
 // to pick which language's chrome text (Strings.java) to display, so a
-// language change made mid-session via the game's own options menu is
-// picked up without needing to reopen either panel. Same
-// off-thread-scalar-read reasoning as the two getters above.
+// language change made mid-session via the game's own options menu (or our
+// own Language row, or Load Configuration) is picked up without needing to
+// reopen either panel; also doubles as the Main Menu tab's Language row's
+// own selection highlight, same as qs_config_current_renderer_is_opengl()/
+// qs_config_current_resolution_index() above. Same off-thread-scalar-read
+// reasoning as those two getters.
 int qs_config_current_language(void);
 
 #ifdef __cplusplus