I suspect real-time machine translation is within reach if the speakers are trained to use a restricted subset of their native language. Moreover, I suspect that this approach could be both realistic and useful. This restricted subset situation is de facto already the case; existing tools don't handle regional dialects and colloquialisms. I just tried "嘘じゃねいぞ" (uso ja nei zo!) in Google Translate and it came up with…
It's interesting to note that before machine translation, there have been attempts to try to standardize restricted subsets of English as a way of making it easier for non-native English speakers (for example "Basic English" and "Basic Global English").
However, I think both in the case of communication with non-native speakers and in the case of machine translation, it's actually not that easy for native speakers to anticipate what phrasings will be problematic.
These existing efforts have had the problem that reducing vocabulary can actually make the resulting language more reliant on things such as verb colocations that are actually harder for non-native speakers. Machine translation has an advantage that it can presumably understand a rich vocabulary, so it might be possible to get somewhat better results by using as precise words as possible while keeping the sentence structure simple. Still, especially with modern statistical machine translation, and perhaps even more so in the future with recurrent neural network-based translation, it may not be that easy to predict potential pitfalls in an attempt to work around them. It actually might have been easier to train people to get better results (at least for specific language pairs) with older, rule-based translation systems, since they were more predictable.
In either case, there presumably needs to be a certain level of accuracy before people can start attempting to tweak it by their choice of wording. I don't think we are there yet, especially for English Japanese. I just tried entering "I'm not going to tell him that." in Google translate, and it gave me "私は彼のことを言うつもりはありません。" ("I'm not going to mention him.") This is a fairly simple sentence, with no colloquialism, and yet Google Translate fails utterly. This is much more concerning than something that can be easily worked around, such as not knowing colloquialisms such as ”じゃねー".