Incorrect font rendering in Portal and Portal 2

**Game information**
Portal and Portal 2

**Distribution name and version where applicable**

```
NAME="openSUSE Tumbleweed"
# VERSION="20181208"
ID="opensuse-tumbleweed"
ID_LIKE="opensuse suse"
VERSION_ID="20181208"
PRETTY_NAME="openSUSE Tumbleweed"
ANSI_COLOR="0;32"
CPE_NAME="cpe:/o:opensuse:tumbleweed:20181208"
BUG_REPORT_URL="https://bugs.opensuse.org"
HOME_URL="https://www.opensuse.org/"
```

**Flatpak info**

```
$ flatpak --version
Flatpak 1.0.5
$ flatpak --gl-drivers
nvidia-415-22
default
host
```

**Problem description**

Portal 2 shows substitute characters instead of text in Russian language.
Portal shows substitute characters with any language selected (tested English, Russian and Simplified Chinese) in menus and text boxes, in-game console renders english text correctly.
https://imgur.com/a/m07eX42
Terminal output shows no info related to this problem.
Latin text in Portal 2 is rendering correctly. Other Source-based games, such as CS:S, CS:GO and TF2, are not affected.
I tried to rebuild font cache inside of the flatpak environment and in the system but that didn't helped.

**Does this issue reproduce with native Steam**

Both games work fine in native installation of Steam.