Newly installed BGEE. I installed a few tweaks from cdtweaks-v9, journal mod from EEUITweaks, and all the AI upgrades from SCS; no beta items were installed. During the tutorial I clicked on the SCS difficulty button. This did nothing but send me back to the tutorial, and after that all conversations and responses were Invalid. I had to terminate the game to exit.
I'm going to reset the game, but since it takes forever to mod everything again I'll wait until I hear back from someone as to what could have gone wrong.
This may be addressed by
I'll reload everything and follow their advice, although I know I selected the language correctly.
Reinstalled everything (BGEE, cdtweaks, skipped EEUI, SCS. Works now, thanks for the mod.