hugo/docs/content/en/functions/relLangURL.md

title	description	date	publishdate	lastmod	keywords	categories	menu	signature	workson	hugoversion	relatedfuncs	deprecated	aliases
relLangURL	Adds the relative URL with correct language prefix according to site configuration for multilingual.	2017-02-01	2017-02-01	2017-02-01	multilingual i18n urls	functions	docs parent functions	relLangURL INPUT				false

absLangURL and relLangURL functions are similar to their absURL and relURL relatives but will add the correct language prefix when the site is configured with more than one language. (See Configure Languages.)

So for a site baseURL set to https://example.com/hugo/ and the current language is en:

{{ "blog/" | absLangURL }} → "https://example.com/hugo/en/blog/"
{{ "blog/" | relLangURL }} → "/hugo/en/blog/"