Starting with WordPress 4.3, you can use languages such as "Deutsch (Sie)" (formal German) in your WordPress installation. But if the themes or plugins you are using do not have a translation file for this language, WordPress would use the default language, usually English, instead. With the help of this plugin, you can set a fallback for your chosen language. Every time a translation file is loaded, the plugin will then load the fallback if a translation for the originally chosen language was not found.
A list of all my plugins can be found on the WordPress Plugin page on my blog kau-boys.com.