Hi @vpejcoch,
The only way is to use Drop-in. Is that somehow supported by some plugins?
Drop-ins are PHP files on specific locations that get included if they exist. The inclusion is done for a specific task – in this case what you are looking to do with translating BrainTree. If BrainTree gave you a Drop-in and instructed you where it is to be set into your site, it can be installed with help of your Host.
More information on Drop Ins and Must Use plugins can be found at this link.