The issue happens when I select French or German version and submit the membership form. Membership form is using Fluent forms. The WordPress user backend fields are mapped using Metabox custom fields.
The integration between Meta Box and Fluent Forms is maintained on the Fluent Forms side. So I recommend contacting Fluent Forms support to get further assistance.
I also see the select field in the admin area is a text field. You can change it to the select field and see if it helps.
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