I'll start by admitting that I am a typical American who only speaks one language.
I maintain the PL./Haskell extension (https://github.com/ed-o-saurus/PLHaskell). I recently received a bug report from a user who is unable to load the extension when they have set lc_messages to something other than 'C' or 'en_US.utf8' (https://github.com/ed-o-saurus/PLHaskell/issues/9). I'm not sure how to address this issue. I know it has something to do with NLS, but I'm confused about what I need to do to get an extension to work. The documentation in chapter 57 of the user's manual seems to address stand-alone programs. At least that is my understanding.
I am using my own makefile, not the one from pg_config.