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