I wanted to write a little about my experience of getting lyrics to display in Poweramp. If anyone has the same problems as me, maybe they'll find this through a search engine.
I was trying to write Japanese lyrics to id3 tags in bulk on a Linux system. Firstly I found that there are a plethora of tag editors, but some don't include unicode support. I also wanted command line support to allow for bulk writing with a for loop. The tool that worked for me was eyeD3, which I downloaded via pip. It has an option to add lyrics from a text file, but I had some problems initially. This option writes to the USLT frame that is read by Poweramp. The USLT frame takes the "description" and "language" options, and I found that setting the language as Japanese resulted in the lyrics not being displayed. After that, running the "add lyrics" option without specifying description or language (so that it uses the default English) worked for me.