Transitioning from languages to neuroscience/cognitive science for PhD, how to increase chances of success?
I studied at Edinburgh with a few friends who had come for the arts world into CogSci and CS Masters courses, they shared many lectures with us (Bio-Informatics).    I would suggest either doing a Masters in a related field, before jumping into the PhD.

I took some CogSci and Neuroinformatics modules for my MSc and I struggled not coming from that background, if I were doing it again and wanted to prepare I would look to the online courses. Many of them are excellent.

MIT have an Opencourseware section on CogSci: http://ocw.mit.edu/courses/brain-and-cognitive-sciences/
No one will look down on you for saying you have worked through an MIT course. They also have extensive CompSci resources. Coursera also has many excellent course, that will help fill in gaps in you education.

I don't think you will find many people in academia who will look badly on people using any resources they can find to expand their knowledge and understanding of a given field.