The Marquesans speak a dialect of Tahitian. Don't know if there is any on line or other source to learn Tahitian. If you pick up their language you will be treated way better than the typical French or
English speaking cruiser/tourist. The father of an American
family that was cruising when we were there had cruised to
Tahiti with his dad when he was around 10 years old. They'd put him in
school in
Tahiti or Moorea for a year and he picked up better Tahitian than French. Only took him a few weeks after they got to the
Marquesas before he was once again talking with the locals like a native. His son, who was also about 10, made friends readily with local
kids and picked up a decent understanding of the language, as well. They were accepted, even sought out for social gatherings, dinners, etc. everywhere they went as soon as they started talking with the locals in their language.
We loved the Marquesas. If you are anticipating a slow
cruise and think you'll spend quite a bit of time there, try and get a years visa before you leave. That way you won't have the hassle of renewing the visa periodically and possibly having to sail to Papeete to do it. We had a six month visa and used every bit of it and still didn't see all the Islands. We'd also liked to have spent more time in the Tuamotu's but had to get to Tahiti to renew our visa. It's windward sailing to get back so most cruiser's don't make the effort once they've gone west.
FWIW, no one would have ever accused of rushing through Islands.