ouh la! n'utilise pas concubine! ça n'a pas le sens de compagnon en anglais (a woman who lives with a man, often in addition to his wife or wives, but who is less important than they are: The sultan’s wives and concubines lived in the harem.)
People use the words partner or fiancé(e) to refer to somebody with whom they cohabit. Context usually makes the intended meaning clear. I think the use of fiancé(e) to describe a person with whom one does not live, but has an agreement to marry, is in decline.