Changing interface language without multisite

try to not access on super global variables directly, but using filter_input
your code throw a notice if $_GET['lang'] is not set

So your code can be improved:

function toscho_change_language() {
    return 'bn_BD';
}

if ( filter_input( INPUT_GET, 'lang', FILTER_SANITIZE_STRING ) === 'bn' ) {
  add_filter( 'locale', 'toscho_change_language' );
}

Or if your server has PHP 5.3+

if ( filter_input( INPUT_GET, 'lang', FILTER_SANITIZE_STRING ) === 'bn' ) {
  add_filter( 'locale', function() { return 'bn_BD'; } );
}

Finally, if you mind that in future you may want to support additional languages, you can write the function in a easily extensible way:

function gm_change_language( $locale ) {
    $langs = array(
       'bn' => 'bn_BD',
       'sw' => 'sw_TZ' // Tanzania Swahili 
    );
    $l = strtolower( filter_input( INPUT_GET, 'lang', FILTER_SANITIZE_STRING ) );
    return $l && array_key_exists( $l, $langs ) ? $langs[$l] : $locale; 
}

add_filter( 'locale', 'gm_change_language' );

Problem with this code is that clicking an inner link, language parameter is lost. A possible solution is to use a cookie to store the language.

I’ll apply this improvement to last snippet, but can be used also in the previous.

function gm_change_language( $locale ) {
    $langs = array(
       'bn' => 'bn_BD',
       'sw' => 'sw_TZ' // Tanzania Swahili 
    );
    $l = filter_input( INPUT_GET, 'lang', FILTER_SANITIZE_STRING );
    if ( ! empty( $l ) ) {
       setcookie ( 'my_theme_lang', $l, 0 );
    } else {
       $l = filter_input( INPUT_COOKIE, 'my_theme_lang', FILTER_SANITIZE_STRING );
    }
    return $l && array_key_exists( $l, $langs ) ? $langs[$l] : $locale; 
}

add_filter( 'locale', 'gm_change_language' );

And sure, as you said, you need to translate your theme and load the texdomain using load_theme_textdomain.

And, just a note for future readers, to be able to translate strings in a theme, all of them must be wrote using i18n functions.

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