Hitman™ (2016): Crashes with locale error

**Game information**

Hitman™

**Distribution name and version where applicable**

Sabayon Linux amd64 19.10

**Flatpak info**

`--version`
Flatpak 1.4.0
`--gl-drivers`
default
host

**Problem description**

Hitman™ crashes instantly on clicking play in the Feral launcher. The output contains the very damning
```
terminate called after throwing an instance of 'std::runtime_error'
what(): locale::facet::_S_create_c_locale name not valid
```
Tested overriding `LANG` and `LC_ALL` - both as a launch option as well as for the entirety of Steam - with plain `en_US.utf8` to no effect.

**Does this issue reproduce with native Steam**

No