This is no joke, if you set your mouse to left handed in the settings, you can NOT press the “Start” button in the launcher. It would be quite funny if not for the fact I have to go to my mouse settings every time I want to play the game, and it’s a bother.
After changing mouse settings, everything works perfect though, so it shouldn’t be impossible to fix 
Now which hand is best hand
Yes, I have the same problem. It only seems to be with the Tree of Savior’s launcher. On Windows, if you set your mouse buttons as reversed (left mouse button becomes right, right becomes left). I cannot click the button to start the game as nothing happens. In fact neither mouse button will start it all.
It looks like the game launcher is written in flash and it doesn’t accept the reversed mouse button setting in windows at all.
If I change back to regular right handed mouse buttons, it opens fine.
I hope this is fixed, and if key bindings are added in the game itself to allow the left handed mouse mode also.