I'm not on macOS, so the menus will look slightly different, but the structure should be the same. You should have the App/OpenLP menu, then File, View, Tools, Settings, Help. The second item from the top in the Settings menu will be the Language item, which opens up a submenu with all the languages. Autodetect is always on the top. Try setting OpenLP to that, it'll try to figure out what language macOS is set to, and use that. If that's not working, try manually setting it to one of the English settings (I think they're UK, US and South African English) which will be around the same place as on my menu.
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