Since some time ago (before 2015) I was unable to insert accentuated characters using swiftkey, like é, à, ç, û, etc. — I only saw German Umlauts (or accents from the keyboard language).
But the solution is quite simple. You can explicitly enable accented characters for your keyboard, and you will find the whole options when you long-press your keys. There is a pictured tutorial over at the swiftkey support page.