Set C locale when creating a menu item.
Works around macOS bug described in gtk-mac-integration#14 (closed).
The bug can be demonstrated with tests/testactionbar: In a macOS build on macOS 11 or 12 cd to $builddir/tests
and run LC_ALL=de_DE.UTF-8 ./testactionbar
. Click the testactionbar menu and notice that the >
on Deinste is missing.