Default Dutch is active as language, but there is for sure an English version for developers:
www.mollie.com/en/developers
Allthough it is a Dutch make, a lot of payment methods are integrated:
www.mollie.com/en/pricing
And these methods are Worldwide partners. Also a lot of plugins are available for use in webshop, payment forms, ...
I hope you can reconsider your decision, I just bought Forms because you told Mollie would be integrated.
For your interest: Mollie is one of the biggest payment providers in The Netherlands!