autodesk viewer – Forge Viewer_Languages

According to the documentation you can change the language when initialising the viewer. If the language is not found it will fall back to the default which is English.

function launchViewer(urn) {
  var options = {
    env: 'AutodeskProduction',
    getAccessToken: getForgeToken,
    language: <enter your language code here e.g. 'es', 'fr', 'it' ... >
  };

  Autodesk.Viewing.Initializer(options, () => {
    viewer = new Autodesk.Viewing.GuiViewer3D(document.getElementById('forgeViewer'), { extensions: [ 'Autodesk.DocumentBrowser'] });
    viewer.start();
    var documentId = 'urn:' + urn;
    Autodesk.Viewing.Document.load(documentId, onDocumentLoadSuccess, onDocumentLoadFailure);
  });
}