Textastic uses libcurl for FTP and SFTP file transfers.  Unfortunately, the error messages of libcurl are not localized. But, while it would be nice if they were also available in German, I don't think it's really that important.

Sounds good. I've seen similar implementations in IDEs from Borland/Inprise/Embercadero.

Good idea.

Umm, are you sure you're asking this in the right support forum? Textastic doesn't include a PDF viewer. Maybe I'm missing something?