In the git repository, look at the searx/infopage directory, there is one subdirectory per language. For example, the about page in English is here: searx/infopage/en/about.md.

So to add a new language: create a directory with the language code you want to add (ISO 639-1 Code), copy paste the *.md files from the en directory, and you can start to translate the content (the files are written in markdown, with some special sequence in double bracket {{ .... }} : just ignore them).

SearXNG is going to detect the new files when it starts.

Once done, you can submit a pull request.