There’s a German translation by @Mikawa:
I don’t know about other languages. I think @tomasb was working on a Czech translation a while ago. (Edited to add: Found it! Překlad TADS 3 - tvorba textových adventur v českém jazyce)
According to the docs, you can mix source files with different encodings in a project, but cannot have more than one encoding within a single source file.
If the #charset directive is present at the start of a file, it overrides any -cs option that was specified for the compilation. This makes it easy to mix source files that use different character sets in a single compilation.
An individual file can only use a single character set. It is not possible to switch character sets mid-stream with a new #charset directive.
Quotes from: Source File Character Sets
I haven’t tried it myself, but I think since TADS supports Unicode, you might be able to use that and put the different language elements directly into a single Unicode source file.
Here’s a related post by @RealNC: interactive fiction system for utf-8 RTL languages - #25 by RealNC
I think it depends on the interpreter, as Joey said.