[Resolved] apply_filters hook will not translate a string from a foreign language into the website's defulat l...

Hello,
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The apply_filters( 'wpml_translate_single_string', ... ) hook does not translate into the default language, because WPML does not store translations for the default language — it assumes the string you originally registered (or input) is already in the default language.

In your case:
- You saved "Subject en" in the WooCommerce settings. Since English is your default language, this is treated as the source/original value.
- When you call:

apply_filters( 'wpml_translate_single_string', 'Subject he', 'admin_texts_woocommerce_new_order_settings', '[woocommerce_new_order_settings]subject', 'en' );

you're asking WPML to translate "Subject he" into English — which doesn't exist, because English is the original and WPML doesn’t track translations into the default language.

However, when you ask to translate into Russian:

apply_filters( 'wpml_translate_single_string', 'Subject he', ..., 'ru' );

WPML does return the translation, because it's stored in String Translation.

Why This Happens
WPML stores translations from the default language into secondary languages only, not the other way around.

So:
- Default (English) → Hebrew or Russian (Supported)
- Hebrew or Russian → English (not supported via this hook)

Solution / Recommendation
If you're trying to retrieve the translated version based on the current language, use the default string as the source and pass the current language as the target:

$current_lang = apply_filters( 'wpml_current_language', null );
$translated = apply_filters(
    'wpml_translate_single_string',
    'Subject en', // default language value
    'admin_texts_woocommerce_new_order_settings',
    '[woocommerce_new_order_settings]subject',
    $current_lang
);
error_log($translated);

This way:
- You're using the default language string ('Subject en') as the source.
- WPML looks up the appropriate translation for the current language, which can be Hebrew or Russian.

I'd recommend checking these guides that might help you understand this behavior better and find a solution:
- https://wpml.org/wpml-hook/wpml_translate_single_string/
- https://wpml.org/documentation/support/wpml-coding-api/wpml-hooks-reference/#wpml_translate_single_string

Regards,
Carlos