language attribute can be used to set language for Salesforce Scratch org.

In the following example, language is set to French.

Sample project-scratch-def.json file:

{
  "orgName": "Demo Company",
  "edition": "Developer",
  "features": ["Communities", "ServiceCloud"],
  "language": "fr",
  "settings": {
      "communitiesSettings": {
          "enableNetworksEnabled": true
      },
      "omniChannelSettings": {
          "enableOmniChannel": true
      }
  }
}