Hi, I'm JennyK from REAPER Made Easy. 
 If you are using a language other than English on your Mac, you may run into trouble with REAPER. You will need to run REAPER with a US English keyboard layout. This is to make sure that the hotkeys that we teach you will match up with the hotkeys that will work in REAPER on your machine. This might mean you need to add a second input language on your Mac, so I will show you how to do that, and how to switch between them. 

Let's go to the Mac menu with VO-M. Find System Preferences and hit Enter.

[System preferences opens.]

VO+M to go to the menu bar, and VO+RightArrow arrow until you find the View menu. Once there, arrow down once to open the menu, hit K to find Keyboard, then Enter. 

[Keyboard preferences opens.]

VO+RightArrow arrow until you find the Input Sources tab and VO+Space to select that. 
We find a table here with the input sources we have activated already. On my Mac, I have US and Swedish already, I'll be adding a third language.
VO+RightArrow until you find the Add button, VO+Space on that. 

[Another dialog opens, listing all available languages.]

Select the language you want to add. You should choose US English. As I already have that added on my Mac, I'll choose Albanian instead and hit Enter on that.
Now VO+LeftArrow back to the table of input sources and you will find the one you added. 
Next to the Add button is also a handy Remove button, so I'll use that to remove Albanian from my list. 
Now VO+LeftArrow back to the tabs again, find Shortcuts this time, then do VO+Space. 

[I am now in the shortcut category table.]

Arrow down to the Input Sources category.

[I VO+RightArrow from here to a list of  hotkey assignments for switching languages. The two default hotkeys are Control+Space and Control+Option+Space. They are both checked.]

There are a couple of problems here. Control+Space is mapped to pause in REAPER and Control+Option+Space is to perform the default action on something on the Mac so. I don't know what they were thinking. But it doesn't really matter, we're gonna uncheck one of these and change the other one.
Arrow to control option space, interact with it, then VO+Space to uncheck it.

[I uncheck the checkbox.]

Uninteract with that, arrow to the next one, interact again. 

[I select the Control+Space shortcut.]

We want to change the hotkey for this. Its checkbox has to be checked or it cannot be changed. 
Now interact twice and VO+RightArrow once, find the text field where we can enter our new hotkey. 

[I hit the F19 ke, being careful to not hit any other keys.]

Any key you hit now (unless it's a VO navigation key) will be mapped instead. Like an arrow key or escape or enter. So uninteract before you move on.

[I uninteract a couple of times.] 

I'm using F19 here just because it happens to be available on my keyboard. But what should you use? I can recommend control+F3, that's free on my Mac and not being used in REAPER, but I can't tell you for sure. If you have other ideas, try them first with hotkey help (VO+K) to make sure it's not busy doing something else.
And because accidents do happen. There is a Restore to Defaults button in this dialog, so if you have to, use that to start over. 

So now, I can change my input language on the fly with F19.

[I switch between US English and Swedish with F19. VoiceOver speaks the new language.]

We are done in this dialog so close it with Command+W. 

[I close the dialog and return to the desktop.]

We can also use the menu extras to switch language. To do that, hit VO+M twice, find the language bar, VO+Space to open a dialog, arrow to the language you want to use then Enter to select it.

[I switch language to US English from this dialog.]

And with that you should be all set.