Improve: Support default language

[florianm]

This feature would prevent Collect to default to the first language by alphabetical order.

As per Xforms spec:

A default=”” attribute can be added to a to make it the default language, otherwise the first listed is used as the default.

Example for "first listed": English (en) vs Bavarian (bar) defaults to Bavarian.
This also concerns: aar: 'Afar', abk: 'Abkhazian', ave: 'Avestan', afr: 'Afrikaans', aka: 'Akan', amh: 'Amharic', arg: 'Aragonese', ara: 'Arabic', asm: 'Assamese', ava: 'Avaric', aym: 'Aymara', aze: 'Azerbaijani', bak: 'Bashkir', bel: 'Belarusian', bul: 'Bulgarian', bih: 'Bihari', bis: 'Bislama', bam: 'Bambara', ben: 'Bengali', bod: 'Tibetan', bre: 'Breton', bos: 'Bosnian', cat: 'Catalan, Valencian', che: 'Chechen', cha: 'Chamorro', cos: 'Corsican', cre: 'Cree', ces: 'Czech', chu: 'Church Slavic', chv: 'Chuvash', cym: 'Welsh', dan: 'Danish', deu: 'German', div: 'Divehi', dzo: 'Dzongkha', ewe: 'Ewe', ell: 'Modern Greek'.

Implementation

• Select default: The user would need to choose the default language (checkbox? dropdown? default to first in language list = default language?).
• Export default attribute: The XML export needs to set the default attribute.

Related

Central complains about invalid language codes when exporting to XLSForm. Central wants "English (en)", not "English".