"Raj does not like potatoes."
Translation:राज को आलू पसंद नहीं हैं।
This sentence breaks down like this:
/Raj-to/ potatoes/ pleasing/ not/ ARE/ > "potatoes ARE not pleasing to Raj"
हैं represents the plural form, "are."
I don't see any problem, however, by treating "aalu" as singular. That is also possible in Hindi. So है also makes sense. Just make sure that you are referring to potato(es), not to Raj.