Originally reported here: https://phabricator.wikimedia.org/T128295#2074796 but I guess this was an unrelated bug.
I'm still seeing Sp?cial:Search instead of Spécial:Search, for instance: https://wikimedia.org/api/rest_v1/metrics/pageviews/top/fr.wikipedia/all-access/2016/02/22
This makes it difficult to show only mainspace pages, whereby you might loop through the results and remove any pages that start with a namespace specified in the siteinfo: https://fr.wikipedia.org/w/api.php?action=query&meta=siteinfo&siprop=namespaces