[amelius]

I'm looking for an efficient encoding that is guaranteed to not contain certain substrings, such as improper language.

[jzwinck]

You can probably get most of what you want by simply not using vowels. A few non-English ones will still get through like knnbccb and there's also shlt and so on....

[voltagex_]

Define improper language. Does your improper-language dictionary cover every possible translation of every possible "improper" word?