— edit —
Sorry for disturbing…! I was trying to ask you how you made thing work but I just somehow managed to get it working. And I don’t know how I can cancel replying to you in this thread…
Just adding my notes here for someone else in need.
You can check the $TEXMACS_PATH on your device by: Open a TeXmacs editor > Menu > Insert > Session > Shell, and run echo $TEXMACS_PATH in the inserted shell. It is /Applications/TeXmacs.app/Contents/Resources/share/TeXmacs/langs/encoding/ on macOS for example.
In the $TEXMACS_PATH/langs/encoding/ folder, there are several scheme .scm files involved in the auto substitution of ‘ and ’ in pasted texts.
unicode-cork-oneway.scm
("#2019" "'") ; right single quotation mark
corktounicode.scm
("#60" "#2018") ; typographic backquote
utf8tolatex-onedir.scm
("#2018" "`")("#2019" "'")
You can delete those lines or comment them out by adding a leading ; semicolon, and then restart TeXmacs. Now the curly single quotes ‘ and ’ should be kept as is in pasted texts, and won’t be replaced by backtick ` and straight single quote '.