1. The Shopify API is a generic JSON REST API, in theory you can use any language to create an app and interact with Shopify.
2. We provide and support libraries for Ruby and Python. There are plenty of 3rd party API libraries but we do not support them ourselves.
3. Unfortunately not but there is nothing special about making calls to Shopify, I would expect any Http library would be capable. For example: http://stackoverflow.com/questions/3913502/restful-call-in-java
4. Since our API is JSON you can use any language, if you were to use Ruby of Python you could use our official api library otherwise there is no limitation
5. Not at this time