Thanks for the report.
We are using standard zTXt chunk to store PlantUML source (see http://dev.exiv2.org/projects/exiv2/wiki/The_Metadata_in_PNG_files
Sadly, zTXt must be encoded using ISO/IEC 8859-1 which means that Russian cannot be used there :-(
We could use iTXt chunk that could be compressed, but the use of those chunks is not very documented (at least in Java), so we did not succeed (yet) in compressing them.
Another option would be to encode PlantUML source using UTF-7 ( https://en.wikipedia.org/wiki/UTF-7
) when we detect that some non ISO-8859-1 are used. Then we could store the UTF-7 encoded String in zTXt chunk.
So stay tuned, we'll post some message here when we will be ready to test.