Greasy Fork

This is indeed simple and incredibly useful - all I would add is that when you look at the page source, language IDs are non-numeric (English is 'en'), and that adding the worksearchlanguage_id helps cover the way that I browse.

try {
    document.getElementById("work_search_language_id").querySelectorAll("option[value='en']")[0].setAttribute("selected", "");
}
catch (error) {}

Do you mean for browsing fic? I don't intend to cover search filters in this script—this is specifically for posting. I only write in English, but I browse fic in other languages. You'd be welcome to make your own version forked from this to set a default search language, if none of the existing scripts that cover default filters include language.