Is there any file format standard for EEG data?
As far as I know, short answer is no - there is no gold standard for EEG files. But EEG is a time series (amplitude) from several channels. So the most common way to represent it is columns as electrodes and rows as points of time. I think almost all software is possible to read such structure as txt file. But EEG is not only time series. Usually it is important to have additional channel with event markers. It make the situation a little bit tricky. But usually popular software is able to read most of EEG data formats and it's not difficult to write a custom function for reading your data.