This started as a comment in a diary in which the name "Oesterreich" featured. I soon realized, however, that what I had to say was growing ever more off-topic/thread-highjacky and that it would probably bore many who read it and interest a few who didn't. Bad news all round.
So this diary is a bit off-the-cuff and so I'm sure I'll forget to mention or fail to know certain things that are interesting or important. I apologize.
However if you like learning about languages or the vagaries of accents as used in writing or about the idiosyncracies of language and how history has effected them, or even just want to learn how horizontal colons can be used to convey information, click your way past the orange whackadoodle...
No, not that orange whackadoodle, although he features as well.
An Umlaut or trema is a colon (the punctuation mark, not the bowel) placed horizontally above a letter.
Now, I'm not a linguist and I only speak four languages and basic Spanish. I can (slowly) read Cyrillic and modern Greek letters, but they don't use the double dot accent. To be honest I have no idea whether a colon-like symbol in non-European alphabets is referred to by the same word. If so - that's beyond my ken. If anyone can correct or expand on what I say, please do.
Ironically, although there is no 'trema' in Greek, it meant "hole". It came to us via the pits in the dice via French or Dutch.
Either way the dichothomy between peoples not subjected to rule by the Romans and those weren't is striking. Belgians, Dutch, French and Spaniards - all used this symboll in a fundamentally different way than those to the north.
A trema is used to to seperate one vowel from another. An Umlaut is used to change the sound of a vowel, or occaisionally a consonant.