Feature request: Member preference for default language on multi-language site

As a addition to gettext support for static page content support to multiple languages (see #10), I would like to see option where Laddr member is able to select his/her language preference.

It would be also used when sending newsletter #43 or sending project updates on following/participating projects #4, so that Laddr can tell which language template, and which content language to send out.

It should be precondition before newsletter/updates automatic mailing implementation.

Start by reading the related issues #10, #43, and #4 to understand the existing gettext, newsletter, and project-update work. Define how a member selects a language preference and how that preference determines newsletter and project-update templates and content; done when the preference is ready to support automatic mailing.

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