However, to implement standard State and Country picklist, admin need to clean all existing data, existing State and County must be clean with values defined. It will also cause data from all integration and data load must be clean, when inserting and update to Salesforce, the value must follow defined values, otherwise, an error will be thrown, such as FIELD_INTEGRITY_EXCEPTION - There's A Problem With This Country, Even Though It May Appear Correct. Please Select A Country/territory From The List Of Valid Countries.: Country.
With AddressTools Free, this tool will ensure your users enter consistent address region names with an auto-complete pick-list & additional validation features. No need to clean your existing data before implementing, clean as you go knowing all new data is validated.
Once the package installed and configured, you need to install the have extension for each of your user too, check out this document, if you are using Google Chrome, here is the extension.
They also provide the same functionality through VisualForce pages without the need for a browser extension, however, this feature is reserved for premium users.
At this moment, AddressTools only support in Classic, but I believe they are working on a Lightning release.
Type: Managed Package; AppExchange
Blog last updated: 25 August 2017