I am using PolyLang plugin to make my website multilingual. And I am using Newsletter Plugin for subscribers.
How can I translate strings in Newsletter Plugin?
On current version we do not support multilanguage plugins, sorry. You can create a custom form, one for each language. Shortly we introduce the by-list-messages so you can eventually address subscribers in different language to different lists.
About nrewsletters, when you have lists dedicate to different languages, you can send two different newsletters.