Hi sgberlin,
the current version of the plugin does have language files for dutch in the lang/ directory. It should automatically load the translations based on the setting you have made in the WordPress General Settings.
I have the WP setting in English, because I made the site for a company in the Netherlands. What do I do to change the names of weekdays into Dutch?
You could either try to rename the dutch language file in the lang directory to opening_hours-en_US.mo or for the Overview widget you could use the filter ‘op_overview_widget_weekday’. It will give you the already translated weekday string in your wordpress language (English in your case) and in your filter function you can then return the dutch translation
Anyway I wonder that you don’t get into trouble with translated strings in the theme or other plugins. I believe you better set the WP setting to dutch and use a different solution to display the WP admin in english. Here’s an article on that: https://amigotechnotes.wordpress.com/2013/12/14/different-languages-in-wordpress-theme-and-admin/
