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Yep looks like a bug. You can workaround this by adding a custom translation file to your solution like this:
<?xml version="1.0" encoding="utf-8"?>
<languages>
<language name="svenska" id="sv">
<frames>
<frame name="_blank">
<description>Öppna i nytt fönster</description>
</frame>
<frame name="_top">
<description>Öppna i hela fönstret</description>
</frame>
</frames>
</language>
</languages>
Should be fixed in EPiServer.CMS.UI 12.22.0: https://world.optimizely.com/documentation/Release-Notes/ReleaseNote/?releaseNoteId=CMS-28026
We discovered that the dropdown for link target (Open in new window/Open in current window) has become translated to the same thing (Öppna i nytt fönster/Öppna i nytt fönster). It does however render the correct _target-attribute, so it's just the translation that's wrong.