Editing his profile, an user could add/update/remove tags in the section "Interests" using any language pack of his choice.
When using others language packs then English (en) or Spanish (es), the div.fcontainer within fieldset.id_moodle_interests lacks the YUI id used to trigger the field visibility when the section is expanded.
STEPS TO REPRODUCE
1. Change your profile language to English (en) or Español (es)
2. Go to Dashboard > Preferences > User account > Edit profile
3. Expand the section "Interests" and you are able to insert a tag.
4. Remove all existing interests and update your profile
5. You are able to edit your interests in this language pack
6. Change your profile language to Português - Brasil (pt_br)
7. Perform steps 2 and 3
8. You are NOT able to edit your interests in this language pack
I've found that when using Português - Brasil (pt_br) the "div.fitemtitle label" has a incorrect "for" attribute; and "div.fcontainer.clearfix" completely lacks the YUI id.