Document/WebSite&erp5_web: introduce web site language priority convention to fix report language issue on renderjs ui

1. [Browser is supposed to send something between 0 and 1]( https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Accept-Language )

2. [Localizer will set 2 or 3 for cases where user selected language]( https://lab.nexedi.com/nexedi/erp5/blob/08be2f8cc936fbdc3d5bafb4355c3a475198abc1/product/Localizer/Localizer.py#L149-172 )

3. all print scripts seems set selected language priority to 10

so here we introduce a convention that site language priority should be 8(>3 and < 10) to fix report language issue