Hely there Johannes,
here is a direct copy from our Support page regarding the Squarespace elements which you wish to translate. You will need the script from the link.
 

If you are using Shopify, you get access to the underlying theme files to edit the translations and can use the Multilingualizer formatting to make these pieces of text bilingual/multilingual.
With Squarespace templates, you *can* get access to these files as a developer but it’s non-trivial. There are hard-coded pieces of text such as error-messages on forms, placeholders for search boxes, form titles etc.
To translate these pieces of text, please add the Squarespace Extensions file to your footer. You can find the extensions file here:
https://www.multilingualizer.com/mlcode/squarespace.extension.english.french.txt
Note: The above extensions file includes translations for most Squarespace templates from English to French. For other languages, you will need to modify the translations yourself. If you are using English and French, you can just paste the code above into your footer (below any other scripts you may already have).
Finally – you can extend the hard-coded translations script yourself if you find your theme has extra pieces of english-only text we’ve missed.
In the Squarespace extensions script, on the line following the last line which begins like this:

jQuery('*').replaceText(.......)

Add a line like the following:

jQuery('*').replaceText(escapeRegExp("Enter hardcoded text to translate here"), "Enter translation here");

You can repeat as necessary for any other pieces of hard-coded text that are missing from your specific theme.

In case of any further issues, please have a look at our main support page or our known issues page – your problem will most likely be described there together with an explanation on how to go around it.
Kind regards!