Yes — the languages you add to your project can be used by both your standalone feed and your widgets.
Your standalone feed picks up added languages automatically (visitors can switch, or be redirected based on their browser language). For widgets, the content loads in your project's default language unless you specify one: pass the lang option in your widget config (for example lang: "fr"), or use lang: "auto" to match each user's browser language.
See Multiple Language Widget Setup for the details.