I need the english interface but my signature files of certain accounts may contain iso.8859-1 characters the signature is only appended to the message body (auto or ctrl+g) when LC_ALL pt_BR.ISO8859-1 is set, LC_LANG is not enough, without it only the signatures not containing 8859-1 chars are appended
This is a default encoding problem. Claws can't guess what to use if your locale is, for example, en_US as this language's charset is historically ASCII, so Claws Mail uses UTF-8 encoding. You can either specify an LC_ALL=en_US.ISO8859-1, or convert your .signature files to UTF-8. Check in Help/About (Scroll down the Info tab) to see which charset Claws uses for interacting with files on the filesystem.