"Eastern languages outside of China" - except for Japanese and Korean, which were listed. India has the benefit of widespread English use across all its language communities. The lack of a widely spoken language like Farsi on the list argues AGAINST your claim that this list is targeted to cultures of interest to the NSA.
I'm just not sure I see any relevance to Prism at all here. Does Skype have to offer instantaneous translation in order to permit multilingual transcription of calls? No - if PRISM has that capability, it has that capability, independently of translation being provided to users.
I guess you can argue that if people are using the translation service, they will maybe speak slower, and repeat and reiterate what they are saying until it is communicated clearly by the translator, so the quality of the transcript may be higher - that's maybe valid, but obviously only applies to conversations that are actually mediated by the translator. A skype conversation in Arabic becomes no easier to transcribe by virtue of the existence of this service. Creating this service, in fact, potentially reveals the extent of their speech-to-text capability - unless they have a greater capability behind the scenes, in which case - again, what does this have to do with PRISM? And if you approach this with a paranoid mindset, surely in theory this technology makes it EASIER for non-English-speaking bad actors in countries of interest like Syria to communicate and recruit vulnerable English-speakers in the US, which strikes me as counter to the interests of homeland security.
A more positive view of this would be that anything which permits easier communication across language barriers facilitates global understanding, trade, co-operation and cultural exchange, and that long term that is the only thing which is going to end the cultural clash which drives terrorism and global conflict. So in some ways, it would be nice if this was part of a long-term US foreign policy initiative aimed at making the world a more harmonious place.... but I think it's more likely that it's just a way for Microsoft to expand the market of their product.