There's an unorthodox way to fix this temporarily until MS release some kind of official patch/fix. It simply involves changing your region format, which only really affects how your date/time is displayed and the default dictionaries. Follow the steps below:
Open your Control Panel, you can do this either by right clicking the windows logo that opens the start menu and selected control panel or by pressing the "windows key+ x" then selecting control panel.
Once the Control Panel window is open type region into the search bar located near the upper-right of the window. Click on Region in the search results.
This should now open a new window; the new window should be titled Region and will have three tabs to choose from.
Ensure you are on the first tab - Formats. There will be a drop down menu titled Format:, click on this and scroll down to English (United States) and select this option. Click on Apply and restart your computer.
That's it done! Your Mail app should be working again as normal. You may find that you may need to re-add your accounts but because the Mail app synchronizes with your chosen mail provider no data loss should have been incurred.
This has worked for me and I cannot take credit for the "fix", someone in another forum figured this one out as a stop gap until MS works out the bugs.