Can't set the locale (man-db.cron)

Everyday I am receiving an email with the following content:

/etc/cron.daily/man-db.cron: mandb: can't set the locale; make sure $LC_* and $LANG are correct 

When I check the results of of /etc/locale.conf, I see the result is set to

LANG=en_EN.UTF-8 

When I run the command locale I see the following output:

-sh-4.2$ locale LANG=nl_NL.UTF-8 LC_CTYPE="nl_NL.UTF-8" LC_NUMERIC="nl_NL.UTF-8" LC_TIME="nl_NL.UTF-8" LC_COLLATE="nl_NL.UTF-8" LC_MONETARY="nl_NL.UTF-8" LC_MESSAGES="nl_NL.UTF-8" LC_PAPER="nl_NL.UTF-8" LC_NAME="nl_NL.UTF-8" LC_ADDRESS="nl_NL.UTF-8" LC_TELEPHONE="nl_NL.UTF-8" LC_MEASUREMENT="nl_NL.UTF-8" LC_IDENTIFICATION="nl_NL.UTF-8" LC_ALL= 

Now I see that the LC_ALL is not set but when I set it using the following command:

-sh-4.2$ export LC_ALL=nl_NL.UTF-8 

and then run the command locale again

LC_ALL=nl_NL.UTF-8 

you will see that it is set but somehow when I go out of SSH and check sometime later, I will see again that it is not set and I keep receiving the email.

My question is, how can I solve the locale issue so I don't keep receiving the emails from man-db.cron.

I am using CentOS Linux release 7.1.1503 (Core).