Accept single language as well as both. But warn in such case.

Also, detect language of mail to send.

We added a parameter, so we need to add one to the checked number.

The space is explicitly forbidden as part of encoded text by RFC 2047,
the encoding must be broken out of and re-entered as needed.

Make sure the script fails when something can't be downloaded. Also make
sure the old file is not overwritten in such case.

Store just one certificate (the new one), since nikola can not handle
multi-certificate files.

Store the correct one ‒ we had two copies of the old one previously.

If it is installed and enabled.

Closes: #3610.

to ask it to run right away now, not waiting random time. It will be
used by updater under some circumstances.