[Bug + Solution] Broken cyrillic fonts in the Big Picture mode

I've stumbled upon broken cyrillic fonts in the Steam client, they are replaced with a monospaced font:

System information:
- Host: Linux Mint 20.2 Cinnamon with Linux 5.4.0-80-generic kernel
- flatpak: v1.12.2 installed from `ppa:flatpak/stable`
- Steam flatpak distribution: 1.0.0.72
- Steam client info: build Oct 13 2021, 19:47:24, Steam API v020, Steam version 1634158817

**TL;DR of my solution**: copy microsoft fonts into the flatpak runtime used by Steam

Detailed solution:

1. Exit Steam
2. Install `ttf-mscorefonts-installer` on host system and run `fc-cache -f -v`
3. Find what flatpak runtime is used by Steam: `flatpak list --app --columns=application,runtime | fgrep -i 'steam'`
4. Copy the freshly installed fonts into this runtime: `cp -rt /var/lib/flatpak/runtime/org.freedesktop.Platform/x86_64/VERSION/HASH/files/share/fonts/ /usr/share/fonts/truetype/msttcorefonts` (requires root)
5. Start Steam and launch the Big Picture mode, cyrillic text looks good now:

Would it be possible to embed these fonts into the flatpak distribution so the cyrillic text could look nice ootb?