Take this example:
<span lang="en" class="multilang">
<div>A test in English</div>
</span><span lang="fr" class="multilang">
<div>Un test in Francais</div>
</span>

This works in:

• courses pages;
• html blocks.

However it doesn't work (multilanguage spans are removed, hence all content is displayed independently from the current user lang) in:

• event descriptions;
• forum posts;
• footnotes in clean theme;
• ... (possibly in many other places).

If the div(s) are not present, the multilang spans are kept and the filter works.
My filling is that there is a filter function that all the possibly user-generated content pass trough, and this bug refers to such a function.

I have tested with <div> but there could be other tags that interfere with the language filter.