i18nize "Select all that apply" answer header in the widget

Summary:
See rGWcce66a25f6930b24e8dbd6809f3c5963645e967b

Test Plan:
 * run `make all` (aka subperseus) and check
 * http://localhost:8080/preview/content/items/xe222f30b?lang=pt
 * make sure 'select all that apply' is in Portuguese

Auditors: james