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

README.md

@@ -90,6 +90,21 @@ package`/`make apk` pick it up the same way either way. This one copy of
 game assets feeds both the desktop build and the Android/Quest build
 below.
 
+**Note on French/German**: the GOG Enhanced Edition installer only ships
+English MFD (Multi-Function Display - the small in-game screens showing
+the map, inventory, etc.) art resources (`mfdart.res`) - it doesn't include
+`mfdfrn.res`/`mfdger.res`, which French/German need in addition to their
+own text resources (`frnstrng.res`/`gerstrng.res`, which the installer
+*does* include). Without those two files, switching to French/German (via
+`questshock.config.json`'s `general.language` or the in-headset menu's
+Language row - see [CONFIG.md](CONFIG.md)) is refused with an in-headset
+popup rather than applied, since doing so anyway used to crash the game. If
+you also own **System Shock: Classic** on gog.com (which does include both
+files), drop its offline installer into `res/assets/` alongside the
+Enhanced Edition one and `extract_assets.sh` pulls them out of it too,
+entirely optionally - it still runs fine with just the Enhanced Edition
+installer.
+
 ## 4. Desktop build
 
 Builds and runs Questshock natively on the desktop (your dev machine, or