Welsh also uses a circumflex accent to extend any of the vowels, and since both 'w' and 'y' are vowels in Welsh (leading to many jokes by English speakers about words with no vowels) they can have the circumflex accent too. I've had problems in the past finding the alt-codes to generate w or y with a circumflex accent - so those may be unique to Welsh.
From http://symbolcodes.tlt.psu.edu/bylanguage/welsh.html : > Because of the writing system, Welsh places accents on the letters w (phonetic /u/) and y (phonetic /ɨ/ or /i/), which is very unique in languages of the world. These symbols require Unicode support apart from that of other Western European languages.