Simply navigate to the folder where the game is stored:
C:\Program Files (x86)\Steam\steamapps\common\Assassin's Creed Syndicate\videos\
Change "UbisoftLogo.bk2" to "UbisoftLogo.bk2.bak"
Change "pc_TWIMTBP.bk2" to "pc_TWIMTBP.bk2.bak"

Then proceed futher to YOUR chosen language folder; for example mine would be English:
C:\Program Files (x86)\Steam\steamapps\common\Assassin's Creed Syndicate\videos\en\
Change "Epilepsy.bk2" to "Epilepsy.bk2.bak"
Change "pc_WarningSaving.bk2" to "pc_WarningSaving.bk2.bak"
Change "warning_disclaimer.bk2" to warning_disclaimer.bk2.bak"

That's it. Relaunch the game and you'll be instantly greeted to the main game menu.