Steps to Reproduce:
Go to www.wikipedia.org and use either the language selector on the search bar or the "Read Wikipedia in your language" interface. Go to Cymraeg.
Cymraeg is sorted under "S".
Cymraeg should be sorted under "C". As far as I can tell, Welsh's alphabet is sorted the same way as in English.
This appears to be caused by:
<li><a href="//cy.wikipedia.org/" lang="cy" title="Saesneg">Cymraeg</a></li>
"Saesneg" means "English". :-)