Hi,
Putting an anchor as the destination of the href somehow works (flag is inneffective until js is loaded).
<script type="text/javascript" src="http://localhost/wordpress/wp-content/plugins/mlanguage/mlanguage.js?ver=3.0.5"></script>
<a data-lang="en" href="#change_language" class="mlang-menu-link"><img alt="English" src="http://localhost/wordpress/wp-content/plugins/mlanguage/img/den.png" style="outline: medium none;" class="mlang-menu-image"/></a> <a data-lang="fr" href="#change_language" class="mlang-menu-link mlang-active"><img alt="français" src="http://localhost/wordpress/wp-content/plugins/mlanguage/img/fr.png" style="outline: medium none;" class="mlang-menu-image"/></a>
However, I needed to disable to disable visual editor to avoid WP to strip the <script> tag. I solved this installing the “Enhanced Text Widget” plugin and putting the <script> in such a text widget.
(Note: I do not want to modify the templates)