Since I don't really use these libraries (now that I said it, though, I'll probably have some big project that requires both of them) and English is my first language, it doesn't really make a significant difference to me.
The actual error message only matters to me if I immediately know what's wrong. Otherwise, I just search the code for the text of the message and read the code to try to figure out what went wrong.
I tend to think that anything which can be translated should be. It makes sense that there would be limited or no translation support in areas which are enabled and running before the Lang class is loaded, or under conditions in which you might assume that something went so horribly wrong that you can't rely on the application state. I do think developers appreciate messages in their native language, when possible, and that the distinction between an error displayed to developers and one displayed to end users is sometimes blurred when the developer finds the message adequate for at least a portion of the message passed along to the end user.
Besides, the ftp library's use of lang->line() is piped to show_error(), which is going to end up in the end user's face unless the developer takes significant steps to prevent it.
