Drupal core has a method to have something other than the core module handle taxonomy, variable taxonomy_override_selector. It doesn't add the taxonomy forum element in those cases.
i18ntaxonomy alters the forum node forum no matter what along as $form['taxonomy'] is there, setting anything of $form['taxonomy'][$vid], and it's weighted at 5? so anything that wants to avoid it would have to be weighted higher, but if something is supplying the taxonomy forum it wants to run at around -1 so other modules (content_taxonomy, comment taxonomy add, etc.) can perform their alterations.
The simplest way is to check on taxonomy override selector.
A more complex way is to not override the values unrelated to it, like #type, but it's sharing code so that's more complex.