Polly is essentially the opposite of Transcribe, it is text to speech. You have access to a pronunciation lexicon, this can be used to create pronunciations for certain words or acronyms, such as AWS being pronounced Amazon Web Services. You can also use the SSML, the Speech Synthesis Markup Language this allows you to add effects such as whispering, affect the specific pronunciation of words and use emphasis.
Translate is a very simple service, think somewhat similar to google translate, it can read text in large documents and translate it to another language. This can be greatly beneficial if you need to translate large volumes of text for localization. You can even set formality, brevity, profanity masking, and custom terminology for phrases that may be company specific that you don’t want to be translated into the target language. Certain languages may not be compatible with these settings so be sure to keep that in mind when using these options.