Simply defined, a foreigner is someone from another "country". But a foreigner also carries other properties that usually come about from coming from another country, linguistic differences being one such feature. Even someone from the UK can speak English but if he comes to America, the accent(and some local slang) alone can indicate he comes from elsewhere. So-called Chinese "dialects' are a few degrees more separated than American and British English.
My point is based the practical aspects of communication needed to basically live and function in the community/society. In this sense, I believe I'm the correct one here.
One substantial test would be If an interpreter is needed to function in society... the person is not going to get the same level of function and access as a fluent speaker. Another test is whether it is necessary to switch to the "third universal" language to achieve communication. When it comes to "dialects"(which doesn't carry the presumption of intelligibility as when used for English), speakers of different Chinese "dialects" would need such translation assistance in the case one of the parties do not understand Mandarin.
Since the dialects are unintelligible, they can be used to code-switch for "secrecy and privacy" in an ongoing conversation, no different than a Spanish-speaker switching to Spanish from English to discuss things with other Spanish-speakers, and the English speaker doesn't have a clue what they are saying.
For example, my aunt needed her son to translate for her. They live in Hong Kong, thus the native language is Cantonese. She went to a doc a few months ago. The doc only speaks Cantonese. So her son has to translate the doc's words into Mandarin. Her son, growing up in Hong Kong, doesn't know Shanghainese. Meanwhile, while my mom and my aunt converse, it's for hours in full blown Shanghainese.
Does it matter if English now has some French elements from the invasion of the Normans? Not enough to drop an American in Paris and expect him to get by like a Parisian who spoke the language from birth. Or a speaker of one Romance language like French being able to understand Spanish without learning the other language. Yeah, their ancestors all had Latin shoved down their throats but things have deviated from those times.
That there is a common origin in the past would speed up learning the other language to an extent but it still requires the effort of learning all the quirks, intonation, and vocab of the new language. An "accent" will still present itself, as the other language's quirks simply cannot be learned well once someone passes a certain point in time.
"Wu" Chinese, which includes Shanghainese, still has a "v" sound while other "dialects" have lost that sound, for example. Thus, someone coming from a different dialect will have issue with enunciation and it will be noticeable.
The effects are pretty evident, with Wikipedia noting that there is not much an oral political tradition. Also, Mao empahiszed his Hunan origin when speaking Mandarin...making his speech unintelligible.
I'm not the only one who has given the distinction of language and dialect much thought.
https://www.theatlantic.com/interna...1/difference-between-language-dialect/424704/