The English language has a nearly defunct mark similar to an umlaut called a diaeresis. When is it used?
The diaeresis, which is pronounced as "die heiresses," is from the Greek. It means "divide," and the two dots are often mistaken for the umlaut. The diaeresis is completely different from the German umlaut. The umlaut is used to change the pronunciation of a vowel, and it sometimes changes the meaning of a word as well.
'The New Yorker' is one of the few publications that still uses it to distinguish between two vowel sounds in a word when they occur back to back, as in "naive" or "releect," for example.
So why has it's use nearly slipped into oblivion? Because the practical use of the diaeresis is of limited benefit. To better understand that answer, take a look at the word "cooperate." It may also be written as "co-operate," or "coöperate." In reality, most people do not have difficulty in pronouncing the word no matter how it is formatted. As a result, most people don't see much purpose in using a nearly obsolete symbol.
Besides, on many programs today, it can be challenging to get the diaeresis to stay over the correct vowel. It is often automatically taken off by modern technology, such as autocorrect.
Still, it's interesting to learn about how English usage has evolved.