Strings to translate can be in texts/messages resources, they can be hard coded in the script themselves (more common as the game is older) and can be in the views.
In your case, I haven't checked it, but anothet possibility is that the 'both' doesn't exist in any of these resources.
ScummVM adds this button in some games that didn't originally had them.
Just open this game in Dosbox and see if that button exists.
If it's there - it's not related to ScummVM.
If it's not there - ScummVM added it.
Then, I don't remember all the details. They based it on some mechanism that was already in the game. But I *think* that the button view is in ScummVM code.
Edit
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For PQ4 there's a comment in ScummVM's code (
https://github.com/scummvm/scummvm/blob/101e016af176d95a43c31b1fdb1dd8338c409843/engines/sci/resource/resource_patcher.cpp#L147) that says that there isn't appropriate view in the game, so they added it.
But I haven't seen such a comment for other games...