This class can be used to provide an interface for specifying
internationalized text strings saved in message keys via input
from a form. When editing the content provided via the input field
is saved together with an item-specific message keys in the
message key tables via lang::util::convert_to_i18n.
This formfield class is especially useful for xowiki items which
have no language-prefix (e.g. folders or links). In other cases it
is probably still a better idea to create same named pages with
different language prefixes.