In the case of french, say:
- Metropolitan and Canadian might understand each other
- Metropolitan and Sub-Saharan too
- but Canadian and Sub-Saharan not so much
You get used to it after a while and Swiss people will try to meet you halfway and speak a bit less dialect (which then sounds strange to other Swiss people). They have some words that are different but Germans normally get. As with AE and BE, it gets funny when the same words have totally different meanings.
On that note: Swiss-German is one form of Alemannic, a dialect also spoken in South-Western Germany, varying from region to region as it does in Switzerland, but do not call it a dialect, they see it as a separate language.
I'd have have to expend similar effort with someone speaking Jamaican Patois or thick Glaswegian English.
I guess my point is that it's a simple task to label languages if you apply prior knowledge of their historical context, but it might not be the most useful categorisation, either in terms of linguistic similarity or in terms of regional identity of the speaker
African French is very close to metropolitan French because the language is less than 200 years old in Africa. You see the same effect anywhere a language has spread recently, there's much less divergence and variation.
Most Germans are bilingual in Hochdeutsch (Standard German) and a local "dialect". The question of whether a given speech is a dialect or a language is a political one, not a linguistic one.
^Disclaimer: I live in Eastern Europe, I've never met a Black person (I know many dark-skinned people, but they aren't negroes, ergo not Black), I've never even met an American, I may be wrong about the "most"
Most people who do speak somewhat High German are unable to understand a heavy dialect. Somebody who speaks dialect is almost never able to speak acceptable let alone perfect High German. Even people who manage to suppress the accent are easily recognized, because they often build incorrect grammatical constructs (use of different pronouns, use of dative instead of genitive and so on).