Language Selector implementation is stored in Database Settings and used by Manager to selected startup language and save selected language at runtime.
For example, PlayerPrefs Language Selector selects startup language based on system language and stores selected language in PlayerPrefs.
You can define your own Language Selector by creating a class that implements IKnotLanguageSelector. After that you are able to select it in Database Editor Settings tab.
Note
Custom class should have no constructor or at least one public constructor without arguments.
Warning
If you decide to remove, change the name, namespace or assembly of custom class you will get serialization error and possibly lose data. As a temporary solution, add [MovedFrom] attribute to your class before making those changes.