[Andreas]

Warning, dangerous! Backup your Database first.

Assuming that you currently don't have any Hacks installed (it would corrupt them)
[sql]DELETE FROM setting WHERE product != 'vbulletin'[/sql]
[sql]DELETE FROM settinggroup WHERE product != 'vbulletin'[/sql]
[sql]DELETE FROM phrase WHERE languageid=-1 AND product != 'vbulletin'[/sql]
[sql]DELETE FROM phrasetype WHERE phrasetypeid > 150 AND phrasetypeid < 1000[/sql]
[sql]DELETE FROM template WHERE styleid=-1 AND product != 'vbulletin'[/sql]

Rebuild Styles, Languages and Settings (by opening one setting group and just hitting save).
Then post the schema of your language table, as there might be columns that need to be dropped.