As the @todo in the link_validate_url() indicates, the list needs to be completed.

I'd start by trying to map the following letters (look at the .eu allowed list):
http://www.eurid.eu/en/get-eu/why-eu-good-business/domain-names-special-...

But hopefully, the best would be to provide full support to all the IDN chars, as indicated here:
https://www.icann.org/resources/pages/idna-protocol-2012-02-25-en

I'd suggest that the simpler fix for now would be to add support for the .eu characters.