Baseline (Widescreen) Solution & Issues
Ultra-Widescreen (21:9) Specific Solution & Issues
Unfortunately in 21:9 the Menu becomes to big and you cannot start the Game.
Workaround via HEX edit: Go to --> ...\Steam\steamapps\common\Action Legion\ActionLegion_Data\Managed\UnityEngine.dll
Go to Offset: 41C0 look for 00 00 80 3F 22 00 00 80 3F and change it to 00 00 40 3F 22 00 00 40 3F
Now the Menu ist smaller and you will able to see the Mission Start Button. Downside is some Fonts are disapeard but the Game itself plays fine.
Eyefinity / Surround Specific Solution & Issues
Same Method as in UltraWidescreen - lower the Numer 80 3F to B0 3E but the Menu become way to small to play.
4k UHD Specific Solution & Issues
Stereoscopic-3D Solution & Issues