Steps to reproduce:
- Go to Commons and enable the NWE beta feature
- Pick a translatable userbox (e. g. User Wiki Loves Monuments)
- Change your interface language to one of those that the chosen userbox has a translation for (e. g. Ukrainian in this case)
- Edit your user page, paste the template into it and load the preview.
With NWE enabled, you'll see English text in the userbox, even though there's Ukrainian translation available. If you disable NWE and repeat the process using OWE, the preview will work like a charm.