This article summarizes my experience on translating office add-ins to multiple language. It is not an official guide, but just how you could do it. It assumes you have an office extension (VSTO, desktop) in a single language, and want to provide it in some other languages for users in other countries.
GitHub demo repository:
For generic information on localization on application localization in .NET, please refer the official Microsoft documentation: https://docs.microsoft.com/en-us/dotnet/framework/resources/index
The steps I suggest: