Resource providers are responsible for doing read/write operations for the resource files. XLocalizer has built-in support for .resx, .xml file types. Additionally it has support for DB with EF (requires XLocalizer.DB).
Extending source types
You can use any custom file/db type for storing localized strings by implementing IXResourceProvider interface for file based types (e.g.: json, csv, ...etc.) or IDbResourceProvider interface for DB sources.
File based sources
A good sample is the built-in XmlResourceProvider. All you have to do after implementing your custom provider is to register it in startup:
You can have a look at the EFDbResourceProvider, it has been built with EF support, but you can create your own DB resource provider by implementing IDbResourceProvider and registering it in startup: