I'm trying to sing Bach now and the director is constantly asking us, being Dutch, to work on our German pronunciation. Most of it seems to boil down to 'pronunce better so people can hear what you're saying' - we seem to be lax in our pronunciation compared to the Germans. But one particular advice is interesting; in Dutch, the schwa ("unstressed filler vowel") sounds much the same wherever it is used, whereas in German, it adapts to the preceding vowel: for example, in the Dutch words "denken" and "lachen", the two last es sound the same, whereas in German, the first is higher. So German has some very subtle vowel harmony, not nearly strong enough to be exposed in spelling, but strong enough to be different from Dutch.