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
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.