seo: should i add a language code in my html if i only have one language?

You can configure the lang attribute on your label if you want. The w3c recommends configuring it.

If you configure it, the value should be enno en_US. It is a language identifier, not a locale identifier. You should only specify the language, and not the language and country.

However, setting the lang It doesn't really do anything. That attribute is not used in any way as far as I know. Google says they ignore it because they find it more reliable to detect language from the words used rather than relying on metadata.

google USA hreflang tags to target alternative versions of your page to different languages. See However, if you only have one language, you shouldn't use hreflang tags.

If you end up directing your site to the US only. In the USA, Google will not send traffic from other countries to your site. If traffic from other countries is valuable to you, you shouldn't try to target your site only to the US. USA