RFC: New i18n architecture (may not be based on Zend)

[chillu]

Acceptance criteria:

• This cards is updated with a proposed architecture, to share as an RFC

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Alternative to RFC: Upgrade Zend Framework - only considers Zend_Locale,Zend_Translate,Zend_Date. ExcludesZend_Cache`.

Context: #5359 (comment)

TODO Write RFC :) We're keeping this task as a placeholder for our Scrum backlog.

[sminnee]

@chillu I felt that the focus of the title wasn't quite right; re-labelled to shift emphasis.

[tractorcow]

I've drafted up an RFC at https://docs.google.com/document/d/1O3ZLOtIjcV_K4XieyWEHK_akegRvmkcIgPqduuGb5RI/edit#; This will be posted to github pending internal review. :)

[tractorcow]

Closed with #6194