$ cat myapp.scm 
(use free-gettext)

(textdomain "myapp")

(define gt (make-gettext "myapp"))
(print (gt 'locale))
(print (gt 'domain))

(set! gettext (gt 'getter))

(print (gettext "hello world"))


$ cat locale/pt_BR/LC_MESSAGES/myapp.po 
#: my-app.scm:11
msgid "hello world"
msgstr "Olá mundo"


$ LANG=pt_BR GETTEXT_PATH=locale csi -s myapp.scm 
(pt_BR)
(myapp)
Olá mundo


;;; So far so good, but with the compiled .mo file:

$ msgfmt locale/pt_BR/LC_MESSAGES/myapp.po -o locale/pt_BR/LC_MESSAGES/myapp.mo

$ mv locale/pt_BR/LC_MESSAGES/myapp.po locale/pt_BR/LC_MESSAGES/myapp._

$ LANG=pt_BR GETTEXT_PATH=locale csi -s myapp.scm 

Warning: lookup-mo-message: (string-length) bad argument type: #f

Warning: lookup-mo-message: (string-length) bad argument type: #f
(pt_BR)
(myapp)

Warning: lookup-mo-message: (string-length) bad argument type: #f
hello world