Just say Ni Hao or Nin Hao for respectable or more political.

Hi Emmanuel, this is a great question. Coming from a long-time Mandarin learner who has also taught a fair number of Mandarin students, let me say that you're not alone in wondering about this. First of all, it's important to realize that Mandarin speakers do not necessarily start conversations by saying, "hi, how are you?" This is something we do in English, but not always in Mandarin especially with strangers. The same goes with saying nihao; it's not always a greeting used between people who know each other well. Understanding what greetings to use and in what context is somewhat of an art that took me a number of months, perhaps years, to get a grasp on. If you're really interested, when I was a beginner learner I actually made a whole list of ways I might greet someone since there are different ones to use depending on the social situation. Just sent me a private message if you want it and I'll see if I can dig it up. With strangers, you wouldn't really say "how are you," at least based on my experience. Even though it's important to know how to say that in English since we usually start conversations with strangers (and friends/family too), it is not as common in Mandarin. I hope this helps! Feel free to message me if you have more questions or reply to this thread. I'd be happy to help.

You can just say Ni Hao