import { HttpEvent, HttpHandlerFn, HttpRequest } from '@angular/common/http';
import { Observable } from 'rxjs';
/**
 * Intercepts an outgoing HTTP request and modifies its headers to include default values
 * for Cache-Control, Expires, Pragma, Content-Type, Accept, and Accept-Language if they
 * are not already present.
 *
 * @param {HttpRequest<unknown>} req - The outgoing HTTP request to be intercepted and modified.
 * @param {HttpHandlerFn} next - The next handler in the chain to pass the modified request to.
 * @return {Observable<HttpEvent<unknown>>} An observable of the HTTP event resulting from the processed request.
 */
export declare function apiHeadersInterceptor(req: HttpRequest<unknown>, next: HttpHandlerFn): Observable<HttpEvent<unknown>>;