Yes, the plural definite of words on -are is -arna. So here, the citizens would translate to "medborgarna".
Words on -are are the same in indefinite singular and plural (en medborgare, två medborgare) but in the definite the difference is medborgaren and medborgarna respectively.
Does that help? :)
As I understand it, 'medborgaren' is national identity, but does not imply residence (that is, citizenship as in what's put on a passport), while 'invånaren' is more along the lines of 'inhabitant' (someone who lives somewhere, part of the population, etc). Assuming this is correct, it's possible to be a medborgare o a country but not an invånare of it (you're living abroad, but have citizenship in that country), or to be an invånare of a country but not a medborgare of it (foreign national with a residence permit).
Google Translate seems to be preferring the more commonly used definition of citizen in everyday speech here.