In German, Zwieback is pronounced /tsvi:bak/. In English, zwieback can be pronounced /swi:bæk/, /zwi:bæk/, /swajbæk/, /zwajbæk/. What many call long vowels in English ā ī ō ū are technically diphthongs /ɛj/, /aj/, /oʊ/, /ju:/, but one of them is a vowel ē: /i:/. (The "rule" you stated for English doesn't work for some words: e.g., weird is /wiɚd/ and /wɪɚd/.) German ei is a diphthong and is pronounced /aj/.

Ceci n'est pas un seing.