Most of the core Ubercart dev team rarely uses i18n, so we are not always on top of the problems and issues. That being said, internationalization is important to us because it is important to you and the community. I know it's frustrating when you submit a patch or issue and no one makes any movement on it. We should be better at addressing your issues. I apologize. Part of the problem is our approach. We tend to focus on one area of Ubercart at a time. So we may not address i18n issues while we are working on porting, views, attributes and shipping, and then knock out all the i18n issues that have piled up at once. This might be the easiest approach for developers, but it is not the best approach when we consider how it frustrates contributors like yourself.
Thanks for pointing out http://www.firewing1.com/node/27 I / we were completely unaware of it. I think we can easily incorporate some of the fixes suggested there. If you would like to help, you could create a separate patch for each of the issues there and submit them to the 6.x issue queue. Maybe even send me a pm when you're done and I'll see if we can't push them through. Maybe you don't feel comfortable creating patches. That is ok, We also really need for people to test submitted patches. Just search the issue queue for issues in the status "Needs Review". Then download the patch, apply the patch to a test site, and test to see if the patch does what it says it does, and doesn't break anything else.
I seem to remember there being a problem with the suggested method of translating "add to cart", but I may be thinking of an issue that no longer exists.
Assuming Views is in a usable state in time, catalog will be completely replaced by default views in 3.x which will solve the catalog block issue.
I have to say, the switching language issue seems like a fringe case to me. But if it matters to the community, and there is a patch, then we'll include it.