Multilingual: Send emails in the correct language

[stephanbrunker]

If a new comment is processed, several mails are sent by the core. Unfortunately, these mails are sent in the language serendipity runs at the time, so if a commenter gets the site displayed in a different than default language, the author gets the notification also in that language.

Because the changes in #692 include also a lot of email sending, there is an universal fix which means to load the language strings for emailing not into constants but into variables. Unfortunately, that means to identify and pull out the mail language strings out of all the lang files and put them into a new type of lang file with variables so the function can load the right language based on the settings of the recipient.

I am going to fix that after I get the subscription itself running which can only take a few more dozen hours.