Comments are handy thing, but it becomes hard to mention people when you use non-english keyboard layouts.
For example, if I want to mention Jonh, it’s simple as that:
<Shift+2>Jonh hello!
Now imagine I need to mention Максим with Russian keyboard layout.
There is no “@” at all. So I have to switch layouts over and over:
<Shift+Alt><Shift+2><Shift+Alt>Максим<Shift+Alt> привет!
If you add alternative shortcut to open mention popup to “+” it will be much easier:
<Shift+=>Максим привет!
Why “+”? Well… I haven’t investigated varios of layouts, but Google Docs use “+” in their comment popups (along with “@”) and I think there is a reason they do so.
At least if user have numpad, then it becomes 1-button shortcut.