Something wrong with prefixes after upgrade d5-->d6

Ні
Actually I know that 6.x branch was fully reworked and is different from 5.x branch
Also I belive it possible to upgrate to 6.x

So,after my drupal was updated to 6.15 I see all my old nodes have additional prefix.For example,originally node with default language was path/bla-bla,after upgrade - uk/path/bla-bla and leads to 404 error.A node with second language was pl/path/bla-bla, now - pl/pl/path/bla-bla and works normally

I tried to disable language prefix by setting "Language negotiation" to "None".Ok,now all my old pages have right view and load correctly,but how can I switch between enabled languages??

Also I tried to recreate old aliases using the same token - [termalias]/[title-raw].No changes,still have additional prefix.

Please help.