Is it appropriate to ask an author for an English translation of a paper?
As far as I'm aware there is no obligation to provide a translation of a published paper from one language to another. 

I think it would be rude to ask for a translation of a paper. However, it would generally be fine to enquire as to whether such a translation exists, or whether similar work has been published in English.

More broadly, it may be in the author's interests to provide an English translation in order to increase the impact of his or her work. 
If the work is really important to you, you could always pay for a translation. At the extreme end, perhaps where there are a large number of important works in a particular language, you may even want to learn the language.