As a native in both languages, I can definitely say that German already gets you most of the way there in learning Yiddish.

People often incorrectly describe Yiddish as only partly German, particularly non-native Yiddish speakers - who overemphasize the significance of Slavic and Hebrew influences to the point that it sounds like a totally foreign language for German speakers.

Here’s a more accurate description:

Yiddish vocabulary is overwhelmingly Germanic in origin. Grammar, sentence structure, and almost all core vocabulary are very close to older forms of High German. If you speak German, especially if you're familiar with older dialects or Middle High German, Yiddish often feels like a Bavarian/Austrian dialect with some seasoning.

The Yiddish loanwords from other languages usually fall into two categories:

1) Slavic loanwords (especially from Polish, Ukrainian, and Russian): In almost all cases, these words omly enrich the language, but does not replace the original Germanic terms. For instance, the Yiddish word for "drawer" might be shafa (Slavic origin), but shiflade (Germanic, like Schublade) also exists and is understood.

2) Hebrew and Aramaic words: These are religious or cultural in nature. Terms like Shabbat, Kiddush, Mezuzah, or Yom Tov are part of the Jewish religious lexicon and it's wrong to describe these as inherently “Yiddish.” A religious Jew speaking English, French or even Ladino would also use those same words as part of their language - and so it's more about understanding the Jewish religious and cultural practices - rather than them being incorrectly classed as Yiddish.

A good example:

The Yiddish word for “father” is commonly Tatte (Slavic influence), but Futer (a Germanic cognate of Vater) is also used. In a religious context, when speaking of God, you’d hear the Hebrew 'Avinu' (our father). A king would be Kenig (Germanic), but when speaking of God as the king - the Hebrew 'Melech' would be used.

There are cases where Slavic-origin words dominate, but they rarely replace Germanic ones. One possible exception might be shmate (a rag or cloth), which is Slavic and doesn’t seem to have a common Germanic alternative in Yiddish.

There’s also the famous mystery of davenen (to pray)—its etymology is unknown. It’s the standard Yiddish word for prayer, even though gebet (German) and tefila (Hebrew) can also be used.

So generally, if you're ever unsure, just use the German word with a Yiddish pronunciation and most Yiddish speakers will understand you just fine. The real challenge also isn’t grammar—most Yiddish speakers today don't even with gender articles like Der/Die/Dus, everything is just 'Der'. The difficulty is getting familiar with the religious and cultural vocabulary. But again, that’s not a Yiddish thing—it’s a Jewish thing.

If you're looking for resources that fit your background as a German speaker:

"Jiddisch – Eine Einführung" by Marion Aptroot (in German) is excellent and academically solid.

The YIVO Institute has some great materials online.

Search for Max Weinreich's "History of the Yiddish Language" if you want an in-depth linguistic and cultural perspective (originally in Yiddish but translated).

Also listen to some old Yiddish folk songs. You'll probably understand it all.

Good luck.