Hiya,
Launching games through the app is primarily for non-launcher-games, e.g. not Steam, not Epic Games, not Rockstar etc. It works in a lot of cases, but sometimes it doesn't. This is because of how these launchers generally implement DRM and thus it cannot launch the games through the game's folder. If you experience launch issues, try to launch the game without the app & once it is launched, select the game in the app and 'Launch' from there.
As for GTA5 and RDR2, this is likely due to at least GTA5 using BattleEye anticheat. In this case, the app won't work at all if you used 'Internal' mode. Attached & Desktop mode only works on windowed & in most cases Borderless Fullscreen games.
I am feeling clueless about why you can't get Desktop mode to work though.. that's worrisome. When you say that "Nothing gets selected", do you mean that you can see the initial 'Hotkey popup' and are able to use the hotkeys and select text, but there's no window with text popping up when you've done so?
If you can, please activate the 'tool' debug and then go to Appdata/Roaming/GameTranslate/crashdump and send me the debug_tool.log that is created. Thank you for reporting! :)