If you're a typological linguist, a language will have a particular structure related to the order of a subject, object and verb in a sentence, or the way certain sounds are produced. A language might be considered analytic (that is, the sense of a sentence is determined mostly by word order and the use of prepositions) or synthetic (the meaning is determined by the way the words in the sentence are modified - conjugation of verbs and declension of nouns). German variants tend to differ mostly with respect to pronunciation and local nomenclature but their essential structure, particularly as to word order (sticking the verb at the end of a sentence, for example) is basically the same, so I think most linguists would call them dialects. Yiddish is probably at the far edge of a German dialect and is often classified as a separate language although it is structurally mostly German.

Unlike the other German dialects, Yiddish uses a completely different alphabet. Furthermore, Yiddish diverged from German several hundred years ago, when it was actually a dialect of Middle German. There are different versions of Yiddish, dialects, I suppose, but the Orthodox version, standard, is very similar to German in many ways, but it does have some grammatical differences and some very different vocabulary. However, if you understand German, Yiddish won't be too terribly difficult to understand in spoken form.

and used the analogy of Proper Queens english and Appalachian southern english...
Kinda sorta the same but not,...YMMV
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Yiddish is based on one area of German, hundreds of years ago.

Plattdeutsch is also the dialect spoken in Northern Germany.

And since a lot of people from there migrated under Catherine the Great to Russia, where they lived for a century or more and developed in their own way in relative isolation and with Russian or Ukrainian influence before migrating to places like Argentina or Kansas, they call their variety of German Plattdeutsch (although the speakers of that language variety I knew prounced it more "plattdietsch&quot .

But, yeah, the problem with splitters and lumpers is that they have different viewpoints. One looks for commonalities, the other for differences. Which is most important?

Well, in the end it depends on the speakers and what they want. If they view themselves as one community, they're likely to change to be more alike. African-American English in the US did that for over a century. The Czechs and Slovaks had no trouble making do with interintelligibility for decades when they spoke Czechoslovak. Since the 1960s, African-American English has diverged, at times with increasing speed, from "white middle-class English". And in the 1990s it wasn't uncommon for me to watch a Slovak speaker stare uncomprehendingly at somebody speaking Czech to him.

The classic study (to my mind) was published in the '80s and considered a dialect of some Mayan language spoken in Guatemala. The large-ish village or small town was divided by a river, and the two sides considered themselves one town and one 'community.' The field researchers documenting vocabulary, grammar, pronunciation, and transcribing stories and narratives tended to agree--the differences were small. In the 1930s something happened--something that greatly offended one side. In the '60s and then again in the '80s somebody went back for follow up research and by the '80s vocabulary, grammar, pronunciation had changed so much that it was clear from the texts and stories transcribed that younger speakers from one side of the river would have real trouble understanding younger speakers from the other side. Took 50 years of hostility.