Add method to get route by language

[Karmalakas]

Just an idea on how redirect problem might be fixed maybe. Or at least partially. Although this doesn't work - couldn't figure out how to get parent. Without the parent, route() returns just a home route. Maybe someone could take this over, or tell me the approach is completely wrong :) But that issue really needs to be fixed. Otherwise I'll have to search for alternatives and I really like Grav, so it would be sad if I couldn't publish my page and had to ditch a few weeks work :(

Also Page::init() calls $this->id($this->modified() . md5($this->filePath())), but it gets overwritten, because $this->id doesn't exist yet, so added a check in Page::id()

[OleVik]

Parent needs to be instantiated, no? I would assume that this goes into the comprehensive work on improving multi-language, @ricardo118? Both pull requests and related issues address deficiencies in current language-handling.