make all new widgets multi-lingual

[MartijnR]

widgets only

[MartijnR]

• [x] second question: label + sub-label
• [x] constraint message
• [x] labels of the group of select question at the end
• [x] label of last (string) question
• [x] slider widgets
• [x] check 'required' subtle message in last question group (in EE)