I really do apologise for this problem.
This was a problem with the 1.1.3 version of the forum. The first issue is with the installation (the attachments path is detected incorrectly and has a missing slash at some point in the path), and the second is with the error handler, see ticket #YCZ-278-40842. If an error occurs on an AJAX request, the headers which are sent in the error handler were interfering with the response sent informing you of the error, thus causing (in this case) the wheel to consistently spin and not update the user on the conversion progress.
There is one other issue as well, after conversion the first time you load a page you may receive either a server error if emails cannot be sent, or an email to your Administrator email address informing you that an upgrade failed.
This is not the case, it is purely the way the way the response from checking the version suggested by the updater is handled. If we don't want to upgrade to that version, it returns false back to the updater, which returns false back to the automatic updater, which in turn attempts to do that. This will be fixed in the next version of the forum.
In regards to the language pack, unfortunately, this is another issue which was present in the 1.1.3 version of the forum. I am very glad to say that it has been fixed now (that language files could not be used in multiple language directories), and will be fully translated in 1.1.4. This fix has been implemented on the converter (second parameter when loading a language resource) but because 1.1.3 is still using the old language handler & cache manager, unfortunately it doesn't currently work on there yet.
All of these problems have already been rectified in the next version of the forum, and is 200% more stable and optimised already - the main index page and default forum loading approximately 0.1 seconds quicker on a default installation and the default topic loads approximately 0.2 seconds quicker.
If I may, I'd suggest that it would probably be better waiting until this next version is released before converting. It's purely an optimisation and bug-fixing release from this point on, (I've added a new Moderator functionality to allow entire user groups to moderate forums, and a new 'config.xml' for styles) but other than these, it's all bug fixes and optimisation.
Once again, I really do apologise that it has caused problems and fell short of your expectations.
chris [at] get-aura [dot] org