How to add a simple language toggle to the ribbon

Yes. But it would be a lot of work. If you haven't done programming or ribbon modifications in the past, figure on a minimum of about 20 hours; maximum sometime by October. Modification of the Quick Access Toolbar (QAT) is much easier and may do what you want.

You could just use a Word Command on the QAT to get to step 4 above.

Modifying the QAT and Ribbon in Microsoft Word

Much easier to record or write macros switching to the languages (this would likely be part of adding ribbon buttons anyway). Then you could:

• Add the macros to the Quick Access Toolbar (QAT)
• Create a short UserForm that lets you select among languages that you use and put a link to that on the QAT. Create & Employ a Userform

Both of those are much simpler than adding a toggle button to the Ribbon, IMO. Besides, QAT buttons are available in all tabs of the Ribbon.

Here is a primer on modifying the Ribbon effectively: Customize the Ribbon (It doesn't take rocket science)

You could also assign keyboard shortcuts to your macros or userform or to the Proofing Language command and skip the ribbon/QAT altogether.

There is a default keyboard shortcut to get to the proofing language now.

Alt+R, U, L

You could assign Ctrl+Alt+Shift+L to the Proofing Language Command or a macro if you wanted. It is not assigned by default to any other command.