Usually Cretans are very helpful, and some also speak French or Italian or German. So if you go to the bus ticket counter at CHQ and say "Chania", I am very optimistic that someone will guide you to the bus. Same procedure in Chania if you buy a ticket to Sfakia - I think there is only a direct connection so that you don't have to switch busses. It might be helpful to write down for your relatives how 'Chania' and 'Sfakia' look like in Greek letters - and to inform them about the bus time table (only helpful if it doesn't change on a daily basis during the summer).
I believe one even doesn't get lost in a taverna in Sfakia - have a look at the food in the kitchen and indicate what you want. We very much like that often one still can go into the kitchen and choose the meal, although this is less common now. I remember the first time in Crete (about 40 years ago) when we sat down at a taverna and wanted to place an order - no one came and asked. Luckily the Cretan people helped - them speaking no English and us speaking no Greek.