They will only see the English content.

You'll need a different URL to serve the other languages. Could be as simple as ?lang=xx on the end.

I'm confused.
Here Google recommends not to use parameters for different languages [googlewebmastercentral.blogspot.de...]
And jet they use them all over the place [support.google.com...]

Here they say: "Keep in mind that Google ignores all code-level language information, from “lang” attributes..." [googlewebmastercentral.blogspot.de...]
And here they recommend lang attributes: [support.google.com...]

Can someone bring light in the darkness?

Apples and oranges. One thing is "lang" in the query string. The other is about "hreflang" in links, leading to language-specific subdomains or to different forms of the URL itself.

I particularly like
Link: <http://es.example.com/>; rel="alternate"; hreflang="es"
[sic]

google recommends against specifying a language in the url with parameters for geotargeting reasons:

http://googlewebmastercentral.blogspot.com/2010/03/working-with-multi-regional-websites.html [googlewebmastercentral.blogspot.com]

Google generally uses the following elements to determine the geotargeting of a website (or a part of a website):
1. ...
Webmaster Tools' manual geotargeting for gTLDs (this can be on a domain, subdomain or subdirectory level) ...

aside from GWT manual geotargeting, a (sub)domain can be locally hosted if necessary:

http://googlewebmastercentral.blogspot.com/2010/03/working-with-multi-regional-websites.html [googlewebmastercentral.blogspot.com]

2. Server location (through the IP address of the server) is frequently near your users.
...

Thanks for the info!

Geotargeting in GWT is not an option, since 20+ different countries are targeted with 2 different languages and in GWT you can only use one country not language.

http://googlewebmastercentral.blogspot.com/2008/08/how-to-start-multilingual-site.html [googlewebmastercentral.blogspot.com]

If you want to reach all speakers of a particular language around the world, you probably don't want to limit yourself to a specific geographic location. This is known as language targeting, and in this case, you don't want to use the geographic target tool.

What if I dynamically change hreflang=x and point to the page itself in the header depending on the users location?

English version: example.com
<link rel="alternate" hreflang="en" href="example.com" />


German version: example.com (same URL)
<link rel="alternate" hreflang="de" href="example.com" />

Can this work?

The user will get the language (+cookie) based on his IP location.

This is bad from the usability point of view. If I surf from a different country, the website will offer me a wrong language, and in fact I am wondering if I will be able to change it at all (depending whether the overriding preference is set by cookie or by IP).

With regards to which language will Google detect, if you have language only recorded in cookie and not as a part of URL, Google will only see English. So the website will miss on potential rankings on other languages.

What if I dynamically change hreflang=x and point to the page itself in the header depending on the users location?

If you want to do this because of Google, this will not work - Google will still only see English. The only way for Google to see other languages is to have language parameter in URL (either for all languages or only for non-default language).

Ok thanks. Will push this in the IT department.