In Microsoft Excel, when trying to type an apostrophe ' I instead get "è".
Does anyone know how to fix this??

Hahaha... i've had this exact problem. What you're doing is you've accidentally pressed the key combination Ctrl + FN + Shift and this provokes the French language on your keyboard.

Press that combination again to get out of it. Make sure you're out of the cell first.

If anybody has a PERMANENT solution for this (eg. disable different languages through registry), I would appreciate your input.

Found a permanent solution....

Go to control panels, Region & languages, keyboards and languages, change keyboards, Go to French and delete, then click apply

Also go to Canadian Multilingual and delete, then click apply.

Good luck!! :)