Multiple languages-implement new blog in existing site

I love WordPress, and have used it for some time, but only for straight forward blogs in one language.

I am now looking at this challenge:

1. implement a wordpress blog in my existing website (so far so good, editing the theme files has worked like a charm. it’s currently installed in https://www.example.com/blog)
2. the blog must be in 4 languages, just like the existing site. Not just the posts/pages, but also menus, categories, tags etc.
3. I would like comments not be separeted (eg German and English comments may be mixed)
4. in the ideal word, I would login 1 admin panel and edit the posts/pages of the 4 languages at once (so not one for each language
5. If a post/page isn’t available in the chosen language (eg French), I want it to be displayed in a default language (English). The worst solution is if I would have to copy-paste the English texts, tags and title into the French fields in the admin panel. I do not use automated translations.
6. I will and cannot use subdomains

I have of course googled my ass off, and found some options:

1. use qtranslate
2. use wmpl (I don’t mind a commercial plugin per se)
3. use xili
4. or use the multisite option with the multisite language switcher plugin

I am prepared to hack a bit. Lucky me has experience with PHP programming. However, I would like the blog to be able to be patched to newer WP versions without the fear of crippling it due to my hacks.
Obviously, as I am implementing it in my current site, I would like to use it in my advantage:

• the site detects browser language and the user’s preferred language. I want the blog to use that, so a visitor does not need to choose a language again solely for the blog. In other words: I don’t want to display flags twice. I need the blog to listen to the website’s language, and the website needs to find out what the links to the same post are, but in the other 3 languages.
• comments should be placed by logged in visitors of the website (so that means I will not be using the users table from wordpress but the one from the existing site)

Everything I mentioned above is possible if you can program a bit, but what would be the best base to start from? I don’t want to be hacking more than necessary. Needless to say that I don’t need separate themes, as it goes into one existing website.

I also read this, but I think you have more detailed information and experience on this matter.