French here. As a general rule, a person speaking only standard French will not understand pure patois. But then, there are really few pure patois speakers left these days…
For the most part, French dialects can be grouped in 4 groups : Occ dialects, oïl dialects, creoles, and separate language.
Occ dialects are based on the old “tongue of Occ”, which used to be the main language in half of France, until the tongue of oïl was basically forced down their throats. Occitàn is the pure form, and that is comprehensible-ish to me, because I also know Spanish and some Latin, so I can guess my way through about 80% of the words. However, regional variations of occitàn can be nigh-impenetrable, because the vocab has gone through so many deformations and bastardizations as to be unintelligible. For example, I could never make a dent in my grandmother’s Auvergnat.
For someone who only knows French ? Muy difficult.
Oïl dialects, mostly used in the north of the country, are based on the language that became modern French, so much of those are comprehensible if you make an effort, however some vocabulary is still obscure, and the grammar can be weird.
Créoles are, well, créoles : a rough-and-tumble mix of multiple languages and slangs. Very little chance of untangling the mix on an ad hoc basis. Even the Martinique/Guadaloupe créoles, which are something like 95% based on phonetic French are really hard for me to understand, although I’ll immediately grok why this word means that once you tell me it does. I will not make the connection myself. For example, in guadalupean, a young girl is “tifi”. Since I know it means young girl, I can backtrack “tifi” to “petite fille”. Makes sense. Fat chance of me figuring it out of the blue.
And then there’s Alsatian (which is basically a derivation of German), Breton and Basque, which are entirely separate languages. Zero pidgin, zero loanwords, zero similarities. You either speak it, or you don’t. Think “Welsh”