No one knows for sure where Terry Nation got the word Dalek. I've read that he says he got it from a volume of an encyclopedia that spanned entries from DAL to LEK. This seems unlikely; how many encyclopedias cover that much in one volume?
Some Doctor Who sources mention the Serbo-Croatian word dalek "far". It's unlikely Nation borrowed the word from Serbo-Croatian, but let's pretend he did for the purpose of this exercise. This dalek is from Proto-Slavic *dalekú "far, distant", a suffixed form of *dalü "distance", perhaps from Proto-Indo-European *del- "long". The extended and suffixed zero-grade form *dlon-gʰo- became English long - and also linger, from the Proto-Germanic causative *langjan - that is "to make long".