static-php-cli/docs/.vitepress/theme/index.ts

// docs/.vitepress/theme/index.ts
import DefaultTheme from 'vitepress/theme'
import {inBrowser, useData} from "vitepress";
import {watchEffect} from "vue";

export default {
    ...DefaultTheme,
    setup() {
        const { lang } = useData()
        watchEffect(() => {
            if (inBrowser) {
                document.cookie = `nf_lang=${lang.value}; expires=Mon, 1 Jan 2024 00:00:00 UTC; path=/`
            }
        })
    }
}