Cx-tools-mt-not-available now says "Not available for LANGUAGE", where LANGUAGE is the target language. What this actually means is that MT is not available for this particular pair that is being shown.
It makes sense to me to include the source language as well, e.g.: "Machine translation from Russian to Georgian is not available". It's longer, but it's a thing that users keep wondering about: It's a frequent question at workshops, and it's also often reported as "Content Translation doesn't work in my Wikipedia" (when actually it does work in all languages, but MT for this pair doesn't).