You want a dead language that's also amendable...sounds like you'll need a central authority that decides what concepts are worthy of becoming words, and we all read that book in school. Concepts have emotional baggage, and there's some interesting research showing that words for an apparently identical concept in multiple languages has very different emotions attached to it, and that's probably true across different regional dialects as well.
Consider the word "marriage", and legal battles all over the place about whether it's defined as one man-one woman or not. Either you can force that precise interpretation on all users of the language, which is only possible if you alter the way people think, or your language is not static and the meaning will shift over time for different people, so...failure.
I guess it might be possible to version a language, so you can label a document or sentence or a single object as being the 2000-2009 interpretation. That could work for historical legal documents, but for a living language, people aren't going to keep up with new definitions all the time...and again, some people will disagree whether should have a changed meaning or not, so when you're in the pacific northwest when people say "marriage" it's locally assumed they mean the 2010 build, and when people in North Carolina say it, it's locally assumed that they mean the 1980 build. That, and you still have the problem where one central authority defines words. Maybe if you allow language forks that anyone can publish as an authority, as long as they are precisely defined, and all legal documents need to be tagged with what fork and build they're using?