Add translations for German, Dutch, Italian and Spanish.

[cmmouritsen]

Proposed changes

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If it fixes a bug or resolves a feature request, be sure to link to that issue.

Types of changes

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• Bugfix (a non-breaking change which fixes an issue)
• New feature (a non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality not to work as expected)

Checklist

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• I have added tests that prove my fix is useful or that my feature works
• I have added necessary documentation (if appropriate)
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Further comments

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[cybergrind]

@cmmouritsen thank you!