Methinks something went wrong with your recent re-install… That shouldn’t happen.
I would try the following first (with Rhino closed)
Go to Control panel>Programs>Uninstall or change
Find Rhino 6 in the list, right click on it and choose “change”
On the splash screen first choose “Repair”
Let it run until complete.
Then start Rhino. Now look in C:\Users\<username>\AppData\Roaming\McNeel\Rhinoceros\6.0 and see if you have a "Localization folder and that it contains en-US Note this folder won’t appear until you run Rhino.
If that doesn’t work, you might want to check this thread for how to do a registry edit to try to fix the problem: