This is a custom language localization editor for Unity. The developer can create a translation unit which can contain as many languages as required. Each language unit allows the developer to define keys for strings and to assign a language-specific string to each key.
The editor can be accessed here:
This is what the editor looks like for the language unit. Note that for a translation to be available, each translation must contain the matching key and language-specific translation. The two photos below demonstrate an English and Polish translation: