I'll take a kick at this can, but it is pure speculation. Maybe someone can come up with a documented answer.

Firstly, we should understand that "back" and "backward" are both adverbs. Both have a meaning which is "away from the front; toward the back". In this definition, they are synonymous.

The implied verb in "back" and "forward" is *go*, as in "*go* back" or "*go* forward." The statement "*go* backward" is grammatically correct but idiomatically it's not how we would say it in English -- at least, the forms of English I'm familiar with. Native English speakers are more likely to say "*go* back." Look at this [comparative chart of how often "go back" and "go backward" are used](https://books.google.com/ngrams/graph?content=%22go+back%22%2C+%22go+backward%22&year_start=1800&year_end=2000&corpus=15&smoothing=3&share=&direct_url=t1%3B%2C%22%20go%20back%20%22%3B%2Cc0%3B.t1%3B%2C%22%20go%20backward%20%22%3B%2Cc0).

That being said, in English user interfaces we will also use "previous" and "next". This is used more for a sequence of pages, such as Google results pages.

As for French, "précédent" and "suivant" don't have a monopoly, either. I often see buttons like "< Retour" to mean "< Back". And in my Chrome interface right now, if I hover over the back and forward buttons, I get "Réculer d'une page" and "Avancer d'une page". Internet Explorer gives me a mix: "Retour" and "suivant".

At the end of the day, what matters from a UX perspective is:

* Your label is clear. "Backward" meets this criterion. A user would expect to go backward.
* Your label is natural. "Backward" could cause a momentary hesitation, just because the wording is not an established convention.

Hope that helps! Bonne chance !