The Shift+Alt in vivaldi brings up the menu at the top left side. However, this shortcut is also conflicting with Windows's Shift+Alt keys to change language. Maybe try to change the shortcut key for the menu.However, I can't find the slot to change this in the Options. Even though I can change the other keys perfectly fine. Help will be appreciated. As of now, I will disable keyboard shortcuts in Vivaldi.
It is worth filing a bug report for this.
Does your language switch OK if you use Alt Shift (in that order) instead of Shift Alt?
This does not bring up the menu.