Fail to Detect Current Language in function.php

Hi,

I am trying to write a function that requires knowing the current page’s language option. I have done many research and have tried the following functions:
get_locale() – This function always return the default language rather than the language of the current page.
get_bloginfo(‘language’) – Same outcome as the get_locale() function.
pll_current_language() – Returns an empty string regardless of what $value I put (some people mentioned it’s returning a boolean false but I am not sure if that’s the case for me).

Here is my site and the code I’m trying to implement:

function wc_add_to_cart_message_filter($message, $product_id = null) {
    $currentlang = get_bloginfo('language'); //also tried the two methods mentioned above
    if($currentlang=="en_US"){
        $message = 'en_US';
    }
    elseif($currentlang=="zh_TW"){
        $message = 'zh_TW';
    }
    elseif($currentlang=="zh_CN"){
        $message = 'zh_CN'
    }
    else{
    $message = 'null';
    }
    return $message;
}

Really simple function, but just couldn’t get the correct $currentlang value. Any suggestion is greatly appreciated. Thanks in advance!

Allen

https://wordpress.org/plugins/polylang/