The client should provide the format. If a site is designed to have multiple languages, the site devs must have settled on a way to switch between the language

The client should provide the format. If a site is designed to have multiple languages, the site devs must have settled on a way to switch between the languages, and so they already came around with a file/data format that matches their system. Instead of trying to guess, the best thing for everyone would be to have a copy of an existing language file and translate the content from there. PS: same for apps, of course.