I use "polish (programmer)" keyboard layout in Windows. While typing commit message I can't use some polish diacritical letters: ć ł ó
To type these letter, we use [Right Alt] or [Ctrl] + [Left Alt], but these combinations are already used by SourceTree for some functions like "Commit Selected" (ctrl+alt+c).

These letters are working in "Options" modal window.

Best option is to change shortcuts to [Ctrl][Key] or [Ctrl][Shift]+[Key].
"Keyboard shortcuts editor" could be also helpful.