the mercurial repo has all language directories changed. I forgot to put it in a "v10" branch so please don't use it for v9!
Why the change?
Well.... I came across multilingual problems.
For example i register on the website in English but the administrator uses Russian.
When he approves your account activation the email that is send to you is in Russian, not in English.
So the system now uses an array() instead of define() to prevent errors when loading 2 languages at the same time ("constant already defined").
The current language system uses [language name] directories and there is always debate what the correct name is. Some directories are from the PHP-Nuke days and those are just misspelled or completely wrong.
Therefore the directories now use the RFC 1766 notation.
Browsers also use RFC 1766 so that's an easy match
Server specs (Server OS / Apache / MySQL / PHP / DragonflyCMS):
Fedora 25 / Apache 2.4.27 / MariaDB 10.1.26 / PHP 7.1.10 / Mercurial
Good move. I encounter that problem all the time (esp. as I'm limited to 2 languages: English and American )
But it is useful to know that the person has registered in a different language. So would it be possible to show in the email or on the admin's view of the member's profile page which language the user has selected - if not that of the main site?
Related: I've never investigated this, but does the new user get the welcoming PM in the site's main language rather than his/her own?
You might be interested in taking a look at PENTAXdslr.eu , although you'd need to register to see the language sub-system. I've extended the multi-language system (with just simple js calls in header.html) so in addition to supporting 5 languages in Forums for posting through DF's own language system, it also handles all languages for reading via Google Translate. Including translating non-English posts back to English so I can monitor those forums. It will all be packaged in reDesign3+ alpha release soon, but could easily be added to any theme I think.