Kerbal Space Program

The Steam API will not be initialised when you start the game via the KSP_x64.exe and not through the steam library but this will not cause your issue. Without the steam API, you are not able to use the steam overlay and proably the workshop integration, nothing else.
This issue sounds more like your localization dictionary is broken, try to delete these two files:

• C:\Program Files (x86)\Steam\steamapps\common\Kerbal Space Program\GameData\Squad\Localization\dictionary.cfg
• C:\Program Files (x86)\Steam\steamapps\common\Kerbal Space Program\GameData\SquadExpansion\MakingHistory\Localization\dictionary.cfg

Than, verify your gamedata integrity through steam (right click on the game in your steam library -> properties -> local files), this will cause steam to download the deleted files again and hopefully fixes your issue.

If verifying the files through Steam does not solve the issue, you might want to check your settings. There's a setting in the debug menu that will make the UI show only the autoLOC codes instead of the localized text. Maybe in trying out things you accidentally selected that option?

The easiest way to check and solve this would be by opening the 'settings.cfg' file in the main KSP directory in Notepad, and look for this entry:

SHOW_TRANSLATION_KEYS_ON_SCREEN = True

If that says 'True', you need to change it to 'False'. Then save and restart the game.