Here is a snippet from the file '/@progress/telerik-angular-report-viewer/dist/es/telerik-report-viewer.component.js' in Telerik Reporting NPM repository of the Angular Viewer that hard-codes the message:
TelerikReportViewerComponent.decorators = [
{ type: Component, args: [{
selector: 'tr-viewer',
template: '<div [ngStyle]="containerStyle" [attr.id]="reportViewerID" #container>Loading Report Viewer...</div>',
styleUrls: [],
encapsulation: ViewEncapsulation.None,
},] },
];
Currently, there is no easy way for localizing or removing this message in an Angular-wise approach.