Diacritical sign of two dots above a vowel (6)

I believe the answer is:

umlaut

(Other definitions for umlaut that I've seen before include "Mark over vowel in German to indicate change in pronunciation" , "Mark over some vowels (in, eg, German)" , "Dots over letter (indicating vowel change)" , "Dots over vowel, as in Koln" , "Mark over some German letters" .)

I've seen this clue in The Telegraph.

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