Rucksack is definitely more common in British English. To my English ears "backpack" sounds a bit American. Even more commonly used in England is simply 'bag', despite it being less specific. Bagpack is used sometimes but I believe it is just a misinterpretation of "backpack".
In UK English rucksack is pretty common, too, and can be used for any sort of bag designed to be carried on the back. Without doing any research, I'd guess it was more common than 'backpack' before the mass influx of US television. There seem to be a few cases where UK and Canadian English are similar. Maybe the Commonwealth lingers :-)
I am Italian but I'm lucky to know German language and therefore I do use a GermanSwedish dictionary as a support. My brain can find out Swedish words more easily when I follow grammar patterns and words' etymology from German rather than English. How are you copying with Spanish?
Really really not obscure in British English!
I wish everyone would add a caveat to their type of English when posting, e.g. 'in Canadian/Australian/US/British/x-accent-or-dialect/etc English, we say blah', so non-native English speakers can learn about what is said where. I forget sometimes myself, and it wasn't meant as a personal criticism- just a general wish.