How do translations work in Craft 3? I currently have a Craft 2 site deployment with a translations folder and a en_gb.php translations file.

I would be able to use expressions like this: {{"visualization"|t}}

However, this does not work in Craft 3. What is the new format for translation in Craft 3?
You can see the changes made to static translations in Craft 3 in the Craft 3 upgrade docs here:

https://docs.craftcms.com/v3/changes-in-craft-3.html#static-translation-files

In your case, craft/translations/en_gb.php would need to change to craft/translations/en-GB/site.php.