What Eva Luna said about Yiddish having a German grammar with some Slavic vocabulary thrown in matches both my own limited knowledge and what many Yiddish speakers have told me. My very minimal Yiddish definitely has a very Germanic grammar; I can understand baby German, particularly in terms of pronouns and verb conjugation. (So I don’t know what we’re talking about, but I know if the subject is you, me or him.) Just as an example, Yiddish conjugates ‘to be’ as follows: Ich bin (I am), du bist (you are), er/zie/es is (he/she/it is), which is very close to the Google translator’s German Ich bin, Sie sind, er/sie/es ist. There are variable amounts of Slavic languages mixed in, depending on which regional variant of Yiddish we’re talking; the German Yiddish dialect has much less than the Polish or Lithuanian. Most Yiddish speakers can understand nontechnical German quite well; they don’t understand Slavic languages that they haven’t learned separately.
FWIW, I’m fairly typically Jewish-looking, but have been mistaken for Dutch by Dutch people, Hispanic by a whole bunch of people, and have been referred to (by my own cousins!) as having ‘an ethnic face,’ whatever that means.