Playing TF2 on Linux is great, but Japanese characters fail to display by default. I found the following font package allows Chinese and Japanese characters to be displayed.
On Ubuntu 16.10:
sudo apt install fonts-wqy-zenhei
Other distros or older Ubuntu versions may refer to this package as “wqy-zenhei”.
The description of fonts-wqy-zenhei states this is a Chinese font, but it works for Japanese characters (Hiragana and Katakana) as well.
Please note this still does not fix the Japanese input issue.