Hello Torsten,
we’re here to help. For most languages, Newsletter is translated with percentages over 95%. That means almost every string in the plugin is correctly translated in that language. However, there will always be a small amount of words (recently added, for example) that will still need translation and approval by the community. We have no control over that: since we’re italians we can only approve italian translations (that’s how the WordPress translation project works).
This regards only the core plugin, which is publicly available. If you installed some of the free addons (which you download from our website), those are not translated automatically but they can be translated via third party plugins such as LocoTranslate (which we greatly recommend). Can you tell me exactly where are you seeing those mixed languages panels or options?
Michael