There are a number of past questions about language prevalence, e.g. Is speaking Portuguese useful in Macau today? or What is the main spoken language in Kiev: Ukrainian or Russian?,
Such questions, however, should still be framed in a way that works for the Stack Exchange question-and-answer format, for example
- They should be scoped to be answerable within a few paragraphs.
- They should provide adequate context, for instance about your intended travel plans. Averages for entire cities or countries aren't really useful, you want the language of street signs, or policemen, or the vendors in such-and-such tourist district.
- They should ask for answers that are rooted in evidence, ideally that can be referenced from verifiable sources, rather than based on personal anecdotes, much less opinions or theories. Questions framed like "how easy?" or "how useful?" aren't really answerable because well-informed people can have different opinions.
- They should demonstrate initial research effort, even as minimal as indicating you've read the language section of the Wikipedia article for some or other locale.
As to your potential question, what is it your relatives plan to do in Prague? How do they plan to get around? Do they know the Latin alphabet? In a European capital in 2018, a Czech phrasebook will get you through basic tourist needs like buying bus tickets or ordering food, but you haven't told us enough to be able to give a useful answer.
The one thing I would say, at this preliminary stage, is that languages being "related" is very far from them being intelligible. Dutch and English are both Germanic languages, but you won't get very far in most English-speaking countries asking the waitress waar is de uitgang?