If you enable language packs after you have removed some out-of-the-box fields (e.g a tab) a language pack will fail. SE is currently working on a hot fix.

 

Workarounds until hotfix is released;

1. Enable for example Swedish before applying the customizations if possible.

2. Re-import your vanilla backup (Always export out-of-the-box entitys before you start customizing)  

  • Export current customizations
  • Import vanilla export 
  • Enabling your languages
  • Re-import your saved customizations

This worked fine for me and I now have a customized system with many languages.

This posting is provided "AS IS" with no warranties, and confers no rights.