"La policía está detrás de nosotros."
Translation:The police is after us.
You are right. This is not English. It is Spanish. And I think that in Spanish, el policía is singular.
detrás de nosotros when translated to behind us (means that the police is supporting us) means entirely different from after us (means that the police is chasing us) How does one distinguish between the two meanings in Spanish?
You're obviously not an English speaker. The police ARE behind us is correct.
It is a feminine collective noun. If referring to a particular person you can use un or una as you would know the gender. Where did you see el policia ? It doesn't sound right.