There isn't much more to explain, the tool just simply does not support UTF-8.
Here is the error message I got:
thread 'main' panicked at /rustc/a28077b28a02b92985b3a3faecf92813155f1ea1/library/core/src/str/mod.rs:660:13:
byte index 20 is not a char boundary; it is inside 'ę' (bytes 19..21) of `Zaproponowano zmianę w termsheet:
*Zapotrzebowanie roczne: 2500000 MWh*
Uzasadnienie: _asdf_`
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
There isn't much more to explain, the tool just simply does not support UTF-8. Here is the error message I got: