Add languages in website

Details depend on the tool you made the website with. But I'm sure you need to translate all content manually.

The best way to have a multilingual website is to translate your site's content manually.

But since you've mentioned adding a function, I guess you want to cut the corner. Do you think of the google translate option? That is free and easy to use, but it does have some limitations (it doesn't always produce accurate translations).

If you're up for a more control over the translations, or if you need your site to be available in multiple languages from Day 1, you'll need a more sophisticated solution. For example, if your website is built in WordPress, you can install the WordPress Multilingual Plugin, which is pretty easy to use.

Choose what best suits you.

follow this steps

1.Translate your website content.
2.Adapt your visual content.
3.Modify your SEO for all languages.
4.Enable your site to be multilingual.

You didn't mention which platform you used to create your website. However, if you want to translate your website content then there are plenty of plugins or software that can do this.
If you are using WordPress then you will find some plugins like Google Translate which will translate your article and you can add a translator option on the site.
If you are using other platforms or HTML sites even WordPress also, you can use some third-party service which will translate your content and even translate content also will index in Google. I can't share the link here but there are plenty and they are available on premium and free also like weglot.
Thank you.