Multilanguage Website
I'm pretty sure your question is about a "multilingual" site, and not really about "multi sites". Consider editing your question (title, tags, etc) accordingly.

Moreover, your question is a variation of these types of questions, and that your issue is actually caused by a language detection issue (something you have not yet configured correctly in your site. Consider updating your question as appropriate. After doing so, I can also update my answer here, to actually pinpoint what kind of admin changes you might have to apply to achieve the desired result. 

Based on the info provided in your question for now, I wonder what the DEFAULT language is that you specified within /admin/config/regional/language of your site. Could it be that it is "es"? If so, and if you also have enabled the "en" language, then change that default to "en" and retry again. Eventually, change the default language back afterwards, to whatever it was before (whenever your Pinterest script no longer requires you to (temporary) change that default language).

You may also want to check your configuration at Configuration -> Languages -> detection and selection: the order should be 'URL, Default, User', and not something like 'Default, URL, User'.