Template string translation support with HTMX requests

[jasalt]

I'm rewriting real-time visitor monitoring functionality using HTMX API WP plugin at https://jyväskylä.hacklab.fi/en/ (visible on top of site), which updates itself every 15 seconds.

The WP i18n feature that Timber theme framework wraps works fine for initial render selecting correct language, but later during HTMX request the language resets to site default language (Finnish). This happened when the hx-trigger polling happens the first time:

Site uses WPML for localization (theme source code here) and these strings have been translated using it's string translation feature.

WPML uses either a query parameter or url path eg. /en/ for deciding which language to render. This plugin using it's own endpoint does not hint the translation facilities about what language should be rendered and it uses site's default language.

[jasalt]

WPML has action hook function that can be used in <partial-name>.htmx.php to override the language before translated strings are echoed that seems to work. Example how I set this up (raw commit here):

Common htmx request context aware function returning language code:

function get_current_language_code(){
	if (isset($hxvals['lang'])) {
		return sanitize_text_field($hxvals['lang']);
	} else {
		return apply_filters( 'wpml_current_language', null );
	}
}

Template code:

<div hx-get="/wp-htmx/v1/visitors?lang={{ get_current_language_code() }}" 
        hx-trigger="load delay:15s" hx-swap="outerHTML">

Controller code for htmx requests (in <partial-name>.htmx.php) to change language accordingly before translated strings are rendered:

if (isset($hxvals['lang'])) {
	$lang = sanitize_text_field($hxvals['lang']);
	do_action( 'wpml_switch_language', $lang );
}
... render HTML after this

Something similar should be possible with Polylang also, I hear that add_action('pre_get_posts', fn(\WP_Query $query) => $query->set('lang', 'en')); should do the trick.