In a political context, foreign mean "relating to different country" where the country from which the difference is measured is determined by context. For example, if, on History SE, you were discussing French history, then foreign would mean "non-French".
But what does it mean in relation to the English language? In the UK, languages such as Gaelic and Welsh are not foreign. By the same logic, Navajo is not a foreign language in an American context. But is Gaelic a foreign language in Canada where it is spoken by only a few immigrants? But those immigrants have been there just as long as the English speakers, so if Gaelic is a foreign language then perhaps English is too.
Even if we know what a foreign language is in the context of one country, what does it mean on an international site such as English Language SE? I suspect many would say that Japanese is a foreign language in this context, but where do we draw the line?
I note there is a related question but although alien and foreign mean much the same thing, foreign is usually understood to relate to a language used in a different country and alien is not usually used in this context.