Why has email stopped displaying mutated vowel sounds correctly?

Hello, I have researched this problem for at least a couple of weeks now and cannot find a solution: using Thunderbird on a Windows PC for a google and a yahoo business account, I have been able to write emails in Thunderbird (with English settings) in both German as well as English with language set in the email to English. The German umlauts used to be displayed correctly but not anymore (perhaps since the yahoo "upgrade"). Yahoo is not helping at all as usual, maybe someone has had the issue and can help. Using the same accounts on a tablet with Outlook works fine in terms of the umlauts, so it might actually be something with Thunderbird after all. I do not know. I have already tried countless times to change from Unicode to Windows and back again. I even had it on plaint text and changed back with the effect that it worked one single time but then stopped working again on the second email I sent. I would really like to get this fixed. Many thanks for any information you might have!