Transcribing non-native speakers of English who are presenters at an international conference?
I assume you mean that the language of the conference is English and that people will be trying to do their best to speak English. 

I think that, while it might be possible that you do it yourself, you are trading time for money. It probably can't be done quickly by a single person, no matter the skills, so a service might be much quicker. I assume that transcribing the talks into "better - more standard" English will require some consultation with each speaker on various subtleties. So, yes, a huge hassle, though do-able if you have the time. 

I think it would be even harder unless you are familiar with the native language of each speaker. 

I think you will also want to get each speaker to sign off on your transcription, however it is done. Some may have questions or even corrections.