This is in some respects a follow up to my previous post in that it has been triggered by our internal project in Ruby. When working in Ruby I consciously lean towards integration tests. Sinatra and Sequel make this really easy as using rack-test and running against an in-memory SQLite database it is easy to simulate a full conversation of HTTP requests and they run reasonably quickly too.
How does this relate to learning a new language? When writing anything in a new language you will make mistakes. You are likely to structure your application poorly. At a minimum you will find there is a much better way of achieving the same result. If you have based your work on unit tests you will have a straight-jacket tying your to your code making the effort to heavily refactor it much greater than it needs to be. Tests will need to be moved, rewritten, thrown away and renamed. However, when you use integration tests you are only verifying the behaviour of the whole system not its moving parts. This means you can replace the internals of your application entirely and your integration tests will not need to be changed and will ensure you have a system that still behaves the same as it did before.
Am I saying that you should never use unit tests in a new language? Of course not. However, you should chose the right level of test to match your level of understanding. Once your application’s structure has ossified it makes sense to exercise the complex parts directly through unit tests. The second you make that shift to using using tests you are setting your structure in stone so you want to make sure that does not happen before it has to.