<globalization culture="en-US">
tag was set in the CMS web config but not the website.
An attempt was made to update this key locally in IIS but when IIS reloads, it reset the culture.
Copying this web config key from the CMS and pasting in the Kentico 12 MVC web.config resolved the issue
Kudos to Arseni Zhdanouski for figuring it out