How to add additional locales?

I would like to use NixOS with the en_FI.UTF-8 locale. However, setting this locale in /mnt/etc/nixos/configuration.nix makes the build of glibc-locales fail with:

Error: unsupported locales detected:
en_FI.UTF-8/UTF-8 \
You should choose from the list above the error.

I am using this locale on e.g. ubuntu, so there should also be a way to get it into NixOS, right?

It does not exist, see localedata/SUPPORTED, which also makes me doubt Ubuntu does somehow support it.

Consider using en_DK.UTF-8 instead. That one does exist and is probably equal in spirit (English with sane units/unit formatting).

Thanks! My guess is that some window managers come with their own locale management, but hard to say.