Can we set the widget language client-side instead of making a separate REST call for each language?

Yes. Fetch one widget URL per fixture, then set the lang query parameter on the iframe URL using a supported language code. Keep all other URL parameters unchanged. This works the same way across all currently available titles and widget types. For example:

const url = new URL(widgetUrl);
url.searchParams.set('lang', 'de');
iframe.src = url.toString();

To change the language after the widget has loaded, reload the iframe with the updated URL. The REST language parameter is still available as an option. The behavior is confirmed in our code, and we're confirming it as an officially documented part of the integration.