[XLIFF] New translations are imported with empty slugs

[timobrembeck]

Describe the Bug

When translations that do not yet exist in the database are imported via XLIFF, the resulting objects have empty slugs.

Steps to Reproduce

1. Export XLIFF file for pages that do not have a translation in the target language
2. Write dummy titles into the XLIFF files
3. Import XLIFF files

Expected Behavior

The translation slug should be automatically derived from the translated title

Actual Behavior

The slug is empty and the translation object is saved with an empty slug