Adding a second input source and selecting Show Input menu in menu bar
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Will result in the display the corresponding flag in the menu bar:
I'm hoping to add a flag to my menu-bar application.
Questions:
No there is not a Unicode character or emoji for each flag at this time. You can set the image of the NSStatusItem instance and change it using setImage: It's a fairly flexible class and you can do other things also for this.
One note, you should take pains to ensure yours is not confused with the input menu. That would be bad for users.