Update the information of the translation page:

Remove the Romain documentation project. The website is down.
Update the Bulgarian URL and remove the mail list.
Update the Traditional Chinese URL.
Update the Estonian URL.
Update the Korean URL.
Remove obsolete URL in the Mongolian Documentation Project.
Remove the Polish URL. The website is down.
Remove the Spanish mailing list. Website down.

Patch by: carlavilla@
Approved by: bcr@
Differential Revision: https://reviews.freebsd.org/D23044