In the past, I was troubled by the garbled characters of Arduino, but this also solves it.
The default font for Ubuntu 18.04 is Noto (No more tofu), but Japanese software that runs on JAVA is usually garbled. Apparently, this is garbled because JAVA's OpenType support is insufficient (probably the side using the software is dragging the bad support of the old one).
sudo apt install fonts-takao-gothic
If you do this, the default font will change to Takao of IPA, which is a TrueType font, and the garbled characters will be cured.