How do I change product option titles to be bilingual (like Color / Size / Title)?

If you have product options like Color, Size or Title, you can make them Bilingual using the Bilingual Shopify Theme.

Step-by-step. Let's do "Size" as an example:

1) Go into the product in the product admin
2) Click Edit Options (the same place where you edit size/color/custom etc.)
3) Instead of size, create a Custom Option. Split the name with a pipe, the same as you would for a tag or a page title. For example "Size|Größe".

From there it will translate, showing "Size" in English mode and "Größe" in German. Same method for Color|Fabe.

Basically, instead of using Shopify's default options for size & colour, create custom ones and re-name them to be bilingual.