How multi languages ​​work

[AndreyChursin]

Hi!
I try to migrate to Bolt from other CMS, and I have several questions about localization.
General component {% include 'helpers/_languageselect.html.twig' %} returns urls with '?_locale=en'
And this setting is not saved between pages and I see other localization only if I have '?_locale' parameter....

Were I can find description for this case?
And how I should setup routes.yaml for "Filesystem based page" with several languages?

about:
  path: /{language}/about        # not work  
  defaults:
    _controller: Bolt\Controller\Frontend\TemplateController::template
    templateName: about.twig

And how it be correct in menu.yaml ?

footer:
  - label: About
    title: About us
    link: /{language}/about   # not work