TO OTHERS WHO ALSO READ THIS TOPIC - please note that your forum can require other rules modifications (used vbseo or other type of vBulletin Friendly URLS) - I made in bold what is essential in modification. So it is 1st rule and modifying standard vBET rules (remember you can have different - because or vbseo, friendly... - just as just described). So do not just copy it and if you have doubts just contact us and we will help you
Please remember that this it modified here only because earlier was used other type of links. If you start with SEO link you do not need anything like that 
OK - issue with old links is solved. It is now redirecting appropriately to new links type.
So this: Harleyforum.se - Skandinavisk forum for Harley Davidson & Brugerdefineret (dansk)
Is redirected here: Harleyforum.se - Skandinavisk forum for Harley Davidson & Brugerdefineret (dansk)
I had to change rules which are described in the post - I think we will have to update those. So your rules now are:
Code:
RewriteEngine On
#TEMPORARY RULES****** - until search engines reindex from old urls to new urls
RewriteCond %{QUERY_STRING} language=(zh-TW|zh-CN|..)
RewriteCond %{QUERY_STRING} !vbetredirected
RewriteCond %{REQUEST_FILENAME} !css.php
RewriteRule ^(.*)$ http://%{HTTP_HOST}/%1/$1? [L,R=301]
RewriteRule ^/?(af|sq|ar|hy|az|eu|be|bg|ca|zh-CN|hr|cs|da|nl|en|et|tl|fi|fr|gl|ka|de|el|ht|iw|hi|hu|is|id|ga|it|ja|ko|lv|lt|mk|ms|mt|no|fa|pl|pt|ro|ru|sr|sk|sl|es|sw|sv|zh-TW|th|tr|uk|ur|vi|cy|yi)/archive/index.php/(.*)?$ archive/index.php?/$2&language=$1&vbetredirected=true [L,QSA]
RewriteRule ^/?(af|sq|ar|hy|az|eu|be|bg|ca|zh-CN|hr|cs|da|nl|en|et|tl|fi|fr|gl|ka|de|el|ht|iw|hi|hu|is|id|ga|it|ja|ko|lv|lt|mk|ms|mt|no|fa|pl|pt|ro|ru|sr|sk|sl|es|sw|sv|zh-TW|th|tr|uk|ur|vi|cy|yi)/$ index.php?language=$1&vbetredirected=true [L,QSA]
RewriteRule ^/?(af|sq|ar|hy|az|eu|be|bg|ca|zh-CN|hr|cs|da|nl|en|et|tl|fi|fr|gl|ka|de|el|ht|iw|hi|hu|is|id|ga|it|ja|ko|lv|lt|mk|ms|mt|no|fa|pl|pt|ro|ru|sr|sk|sl|es|sw|sv|zh-TW|th|tr|uk|ur|vi|cy|yi)/(.*)?$ $2?language=$1&vbetredirected=true [L,QSA]
#END OF TEMPORARY RULES
#RULES for SEO translation links without any SEO engine (on raw vBulletin links)
#If you are using vB 'Mod Rewrite Friendly URLs' then put those rules JUST BEFORE vB rules
#******************************USE THOSE 3 when after disabling temporary rules
#RewriteRule ^/?(af|sq|ar|hy|az|eu|be|bg|ca|zh-CN|hr|cs|da|nl|en|et|tl|fi|fr|gl|ka|de|el|ht|iw|hi|hu|is|id|ga|it|ja|ko|lv|lt|mk|ms|mt|no|fa|pl|pt|ro|ru|sr|sk|sl|es|sw|sv|zh-TW|th|tr|uk|ur|vi|cy|yi)/archive/index.php/(.*)?$ archive/index.php?/$2&language=$1 [L,QSA]
#RewriteRule ^/?(af|sq|ar|hy|az|eu|be|bg|ca|zh-CN|hr|cs|da|nl|en|et|tl|fi|fr|gl|ka|de|el|ht|iw|hi|hu|is|id|ga|it|ja|ko|lv|lt|mk|ms|mt|no|fa|pl|pt|ro|ru|sr|sk|sl|es|sw|sv|zh-TW|th|tr|uk|ur|vi|cy|yi)/$ index.php?language=$1 [L,QSA]
#RewriteRule ^/?(af|sq|ar|hy|az|eu|be|bg|ca|zh-CN|hr|cs|da|nl|en|et|tl|fi|fr|gl|ka|de|el|ht|iw|hi|hu|is|id|ga|it|ja|ko|lv|lt|mk|ms|mt|no|fa|pl|pt|ro|ru|sr|sk|sl|es|sw|sv|zh-TW|th|tr|uk|ur|vi|cy|yi)/(.*)?$ $2?language=$1 [QSA]
Please note that I marked some rules as temporary - after your pages will be re indexed you can remove those add go back to standard rules. It is because you had internal redirection loop even when rules clearly tell that it suppose to be last rule. We had to made little improvisation here (added special parameter to recognize is it already redirected or not. Also I had to made special case for your css.php requests.
Now search engines will appropriately re index your old links to new one. Please note that it can take some time - it depends on your forum size, how often tour forum is crawled by robots and for search engine itself. It can even take 3-6 months before your pages will be re indexed. You can of course just leave all those rules just to be sure. Still those are modified so if you will find any issue please let us note we will help you 
Please check and tell do you need more support here