Plugin does not respect admin user language

Steps to reproduce:

1. Create WP site in a language (eg French)
2. Set up admin user with another language (eg English)
3. Check plugin – it will display in site language (French) in admin, not the user language (English)

This can be an issue on multi lingual sites where admin has one preferred language.

Background: https://wptavern.com/wordpress-4-7-to-introduce-user-specific-language-setting-for-the-admin

Solution: user get_user_locale() to determine the plugin language