Resolve "Missing 2 $field_txt in de_dashlet_limits.lng"

Closes #6246 (closed) Two approaches ...

• For limit_mail_wblist_txt I just add the new string to this specific set of lng files, it does exist in another context but has a slightly different context there 😞

• For unlimited_txt I suggest we also include the global lng file. That one is small enough to not break the template system, and it avoids duplicating the string.