I suggest you to study the IPA: https://en.wikipedia.org/wiki/International_Phonetic_Alphabet
And the German Phonology: https://en.wikipedia.org/wiki/Standard_German_phonology
Should 'yeah' be correct
No; it's considered too informal for this course. Please use the kind of Standard Written English you would use in an essay for school, not the informal language you might use when texting your friends.
Yeah literally comes from the word yes.
And "I'mma" comes from "I am going to", and "wanna" comes from "want to", but those aren't accepted here, either.
"Yeah" doesn't come from "yes". "Yeah" is a respelling of the earlier "yea" (Old English "ġēa"), the exact same word as the German "ja", and "yes" is from Old English "ġēse", where the second element means "may it be", same as German "sei".
To exclude the original and still very widely used "yeah" just because it's "too informal" is just as ridiculous as not allowing the German "ja", since we could use the more formal "jawohl". Frankly, it's dumb.