Locales
By default, the locales for de_CH.UTF-8 UTF-8 and en_US.UTF-8 UTF-8 are generated.
Add Locales
If you want to adjust the list of generated locales, use the locale::locale array within the Custom JSON Server Level Configuration.
The list will overwrite all existing locales, but en_US.UTF-8 UTF-8 is always loaded as it used as system wide default.
The following does add fr_CH.UTF-8 UTF-8 and will remove de_CH.UTF-8 UTF-8:
{
  "locale::locale": [
    "fr_CH.UTF-8 UTF-8"
  ]
}
Tip
You can find a list of supported locales at /usr/share/i18n/SUPPORTED.
Copy the entire line. After update the locales you can check
with locale --all-locales if your new locales are available.