Where can I set environment variables that crontab will use?

You can define environment variables in the crontab itself when running crontab -e from the command line.

LANG=nb_NO.UTF-8
LC_ALL=nb_NO.UTF-8
# m h  dom mon dow   command

* * * * * sleep 5s && echo "yo"

This feature is only available to certain implementations of cron. Ubuntu and Debian currently use vixie-cron which allows these to be declared in the crontab file (also GNU mcron).

Archlinux and RedHat use cronie which does not allow environment variables to be declared and will throw syntax errors in the cron.log. Workaround can be done per-entry:

# m h  dom mon dow   command
* * * * * export LC_ALL=nb_NO.UTF-8; sleep 5s && echo "yo"

I got one more solution for this problem:

0 5 * * * . $HOME/.profile; /path/to/command/to/run

In this case it will pick all the environment variable defined in your $HOME/.profile file.

Of course $HOME is also not set, you have to replace it with the full path of your $HOME.