Umlauts or diaereses?
As far as I know (and the OED agrees..), the two little dots themselves are generally called diaeresis. The use of this diacritical sign in German, on a,o and u, is specific for that language. It transforms these vowels into umlauts, meaning that the resulting sound is only slightly resembling the original one. Other languages, e.g. Turkish and Finnish also make liberal use of the sign, but I don't know it's effect on pronounciation there. In combination with e, the effect is diaeresis, also in German. This use is rare to the point of being exotic.