I wonder if it is possible to change the weight of one word on a tkinter button? So the result would look something like this:
[ yes, I agree ]
I've tried using tags but neither tk.Button nor tk.Button['text'] seem to allow it.
Thanks!
No, it's not possible. If you want a button with rich text you'll have to create your own. Or, create an image that has the look you want, and use the image with a standard button. 
You can create your own using a text widget that is one character tall and a few characters wide. Then, you can place bindings on the button to handle clicks, and to change the relief to simulate a button. Unfortunately, it won't have the look of the platform-specific buttons.
Modifying a Part of Text on tk.Button