The real payload is in the .cab files. For offline installation, you copy the entire ISO contents to a network share or local drive (e.g., D:\Office2019_LP\fr-fr ).
Standard users typically download a "Language Accessory Pack" from the official Microsoft Support site. While the initial download is a small executable ( OfficeSetup.exe ), it functions as a web-based installer that downloads the necessary files during execution. office 2019 language pack offline installer