I am trying to clarify how the translation process is organized in AEM.
I have a blueprint page. I always get a correctly translated language copy after the first running of translation job. Then I add some changes in the blueprint page and update the language copy. The result is also fine. But I need to know how it is translated. Does AEM translate again the full page content of blueprint page or just the changed content? I want to be sure that the page which contains a lot of content wouldn't be translated fully again.
Usually the entire page is translated again. For human translation, the context of the entire page is needed anyway.
The "normal way" to handle that is the "translation memory" of your translation provider. That detects already translated content and well-known phrases. Then the content follows the official translation process, but it comes back instantly from a machine (translation memory).
Problem is the pricing by many translation providers. Some of my customers had such a low price for "automatic"-translations, that they pushed the content again-and-again through the translation process (but was a long-time ago with CQ4, very large company). This is a perfect fit with AEM (or any other CMS). But other clients had no extra price for memory-translations - or only 50% off. In this case the translation providers can have a good initial base price, and earn money in the long run.
Clients with such a "pricing problem" develop many dirty workarounds to avoid translations. Some translation connectors have at least a "Copy Mode". Then only the non-translatable content is copied, and the translatable content is kept (without sending it to the translation provider). This is good, if only some image- or layout-changes were made. Minor text-changes could then be manually translated. I also built technical solutions in AEM, to avoid translations (self-built copy mode, a simple/non-smart translation memory for product descriptions).
As the problem with pricing seems to get bigger today, a translation memory already in AEM would be nice. But I'm not aware of it. All the big translation providers have an AEM-connector (typically built by non-AEM developers, ugly code from hell). And those connectors are just built to shuffle the content to their provider. From Adobe is only the translation process (or translation projects) in AEM.