how to handle internationalisation in web application

My web application would allow users submit content. The application would support 3 languages to begin with but I hope to support more languages in future. Some use case are

1) A user could select to see content of any of the languages (say a German wants to read both English and German content). 2) A user could also submit content in any language (say a German (in Germany) submits an article written in English).

I thought that I could use the Locale information from the browser and tag the content's language. But I suppose it is possible that a German in UK submits an article written in German but the locale information will be UK. Isn't it? If so, I'll end up wrongly tagging the content as English. I suppose another option could be to use Accept-Language but I am not sure if this header is always present in the request

How can I support Internalisation for such an application?