A custom size selector

I want to implement a size switching system on my Shopify product page supporting EU, US Men’s, US Women’s and UK sizing standards. All product variants created in the Shopify admin backend use EU sizes as the base option. Each product needs fully customizable size mapping to define the corresponding US and UK sizes for every EU size, and these mappings cannot be shared between products, as shoe sizing varies by style. Since we carry men’s, women’s and unisex footwear, US sizes must be split into US Men and US Women options. Customers can switch between EU / US Men / US Women / UK sizing standards via a dropdown selector; the size buttons dynamically refresh to show matching size labels. When a shopper clicks any size button, the system selects the underlying native EU variant in the background. A key requirement: the shopping cart must display the EU size alongside its corresponding US and UK size equivalents, Each product also has its own independent size guide content that opens in a popup modal when visitors click the size guide link.