branches.opac_info is text saved in html format. Adding <<branches_opac_info>> in html notices (overdues for exemple) produces letters that are wrong encoded
Sophie, Could you explain more precisely please? I tried with unicode characters in the letter template and in the branches.opac_info => it works I tried with diacritics (é) in the letter template and in the branches.opac_info => it displays "é" for the opac_info. Same behavior for html and non-html letters. Note that the é is contained in opac_info (because of mce). Was it your issue?
Needs verifying, but what i suspect is that the HTML tags used in OPAC info might be displaying, because they get escaped (or were, this is an old bug report).
With TT you might be able to resolve this using raw. Untested.