<?php
$text = '<p> test here [lang_de]German[/lang_de][lang_en]English[/lang_en] and here</p><div> 123 </div>';
echo preg_replace("/\[lang_en\](.*?)\[\/lang_en\]/", '', $text); ?>
PD9waHAKJHRleHQgPSAnPHA+IHRlc3QgaGVyZSBbbGFuZ19kZV1HZXJtYW5bL2xhbmdfZGVdW2xhbmdfZW5dRW5nbGlzaFsvbGFuZ19lbl0gYW5kIGhlcmU8L3A+PGRpdj4gMTIzIDwvZGl2Pic7CmVjaG8gcHJlZ19yZXBsYWNlKCIvXFtsYW5nX2VuXF0oLio/KVxbXC9sYW5nX2VuXF0vIiwgJycsICR0ZXh0KTsKPz4=