Try to format that code in twig format:
<textarea class="form-control" name="email_review_subject_<?php echo $language['language_id']; ?>" ><?php echo isset(${'email_review_subject_' . $language['language_id']}) ? ${'email_review_subject_' . $language['language_id']} : ''; ?></textarea>
So i tried with that format:
<textarea class="form-control" name="email_review_subject_{{ language['language_id'] }}" >{{ isset({'email_review_subject_' . language.language_id}) ? {'email_review_subject_' . language.language_id} : '' }}</textarea>
Error:
Uncaught exception 'Twig_Error_Syntax' with message 'A hash key must be followed by a colon (:). Unexpected token "punctuation" of value "." ("punctuation" expected with value ":")