To quote Wikipedia:
In Dutch, spellings such as coëfficiënt are necessary because the digraphs oe and ie normally represent the simple vowels [u] and [i], respectively. However, hyphenation is now preferred for compound words so that zeeëend (sea duck) is now spelled zee-eend.
Because the diaeresis is used to indicate that two letters should not be read as a digraph, it should be removed when this is indicated in an other way (a hyphen at a line break, for example). There are still cases where hyphenation removes the diaeresis: ‘financiën’ should be hyphenated as ‘fi-nan-ci-en’.
Unfortunately, when using
the word is hyphenated as ‘financi-ën’ (when hyphenation happens at a line break). Is it possible to make XeLaTeX/Polyglossia remove the diaeresis when hyphenation happens?
The language-specific options in the Polyglossia manual do not offer a solution to this problem. The Babel manual mentions the
\@trema macro, and indeed, when using pdfLaTeX and Babel,
financi"en is hyphenated correctly (removing the diaeresis) at line breaks. I did find this ugly hack, but I am convinced there should be a better solution.