easy now, sorry to offend.
There is an unlimited amount of japanese terms one can use, I would go as far to say that the posture that one describes as shisei is not unique to aikido, even if it is a very important aspect. what we teach in aikido is more important that the terms we give it. whether we call it posture, attitude, or whatever, as long as we get our idea across whether we use japanese or not is not the issue.
On a side note, after doing aikido for many years in japanese, I no longer know how to properly explain it in english. I am involved in translating my sensei's writing in english, and it's very difficult. shisei doesn't cause me problems, there are a lot of other words that are much more difficult to translate.
for example: tanden, sabaki, osaekomi, and whatnot
The main point is that we want what my sensei is saying to be accessible to a variety of people, not just those who know a lot about aikido. The longer you practice, sure, you should know these terms in japanese, but for those just starting, it's easier to remember if they use english if nothing is lost in the translation.