In English, the more common spelling of these menu items would be:
Instead of:

If you're up for filing a PR, you can easily edit the affected files directly on GitHub.
https://github.com/glushchenko/fsnotes/search?q=%22upper+case%22&unscoped_q=%22upper+case%22
I dont' have a preference as to how this should be done, but in TextEdit macOS Notes, and Script Editor, Apple uses:
Make Upper Case
Make Lower Case
Capitalize

Apparently Upper Case, Uppercase and Upper-Case are all valid English.
Personally, I use separate words because of the history of the term. Physical letter forms were stored in the upper/lower physical storage cases, ready for typesetting.
Ok, I'll concede. It's best to follow standards set by Apple.
The text editors I use (Textmate and Visual Studio Code) both use the single-word variants. Then, when I saw this n-gram chart, it seemed irrefutable ๐

Thanks for the history, @gingerbeardman, I have a new appreciation for the Cases.
I hold Microsoft responsible ๐
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Ok, I'll concede. It's best to follow standards set by Apple.
The text editors I use (Textmate and Visual Studio Code) both use the single-word variants. Then, when I saw this n-gram chart, it seemed irrefutable ๐
Thanks for the history, @gingerbeardman, I have a new appreciation for the Cases.