So would the literal translation of this be "we are marriage"? Since małżeństwo was "marriage" a couple sentences ago
Nope! Since "małżeństwem" is Instrumental and not Accusative, it's more like "in a marriage" or "under a state of marriage".
After "jesteśmy" you would normally use instrumental. No need to invent some even weirder translations: the literal one is "we are marriage" and the normal one is "we are married".
'We are marriage' doesn't really make sense in English.
małżeństwo can mean either marriage or a married couple
Małżeństwen/ małżeństwo sounds awfully close to the word 'marriage', I wonder if they are cognates from proto indo-European? :)
"marriage" goes back to Latin maritare -- the -age bit is an ending on the root marri-.
Whereas in Polish, the root seems to be małżeń-, małżon- (e.g. małżonek "husband").
My Slovak etymological dictionary says, s.v. manžel ("husband"), that the origin of the older malžen is not entirely clear, but proposes two theories:
1) It may derive from something like *malъžena which would have meant "wife", a part-translation of Old High German māl-wīp "wife", from wīb, wīp "wife" (the word later became modern German "Weib") and mahal "promise, agreement" (compare modern German Gemahl "husband", vermählen "to marry"), with the malъ- bit coming from German and the second bit being replaced by the Slavic žena (compare modern Polish żona "wife").
2) An older form *malъžena might have been derived by dissimilation from something like *manъ-žena "husband and wife" from two roots *manu- "husband" (cf. Polish mąż) + *žena "wife" (cf. Polish żona). Something similar happens in Ancient Greek ανδρόγυνον androgynon "married couple", from ανήρ, ανδρός anêr, genitive andros "man" + γυνή gynê "woman".
So the ż sound is part of the root in Polish but the g is part of an ending in English/Latin.
To my (non-Polish) ears, 'małżeństwo' /almost/ contains the words 'mąż' and 'żona'.
There's nothing in "We are a couple" that even remotely suggests that they are married.
so are there any other alternatives to "married couple"? (would prefer a shorter version if available ;)
In this exercise Duo accepted my "We are married" - which, though a non-literal translation, is correct English (as Vengir has already pointed out).
When I needed to type this in polish, the correct answer was rejected until I put a full stop at the end, seems a bit harsh!
One week later and it is still happening!
Also, I forgot to add before that this discussion was not linked from the question (I got here by searching) which tried to open the first time I clicked it, but the window self closed and then the button stopped working.