There are several approaches to realize a multilingual site in Squarespace.All of the existing solutions have drawbacks. In the answer, you will find a solution for localization of squarespace sites without drawbacks.
1) Official Squarespace Solution
Does not work on all templates.
Structural limitation - you only have one navigation level left perlanguage.
Language selector menu not very nice.
2) Using multiple squarespace sites with language subdomains
You have to pay the Squarespace fee per language 3) Using external localization services like localize.js oder bablic.com
You have to pay for external services.
There are additional resources that have to be downloaded
Does SEO work well on these solutions? Did not investigate this.
4) Javascript based on CSS Selectors and page IDs
Unflexible when adding pages, JS has to be adapted and grows