When piping data from a source to a target wiki using transferbot, an error will occur when the source wiki has content languages that are unknown to the target wiki:

{
  "name": "wikibase-api-not-recognized-language",
  "parameters": [],
  "html": {
    "*": "The supplied language code was not recognized."
  }
}

To fix this, add a custom script to the transferbot image that makes sure only suitable languages are considered.