Hi Imre,
you're right when you say that

> the US and Uk have their army and
> navy on their own, but still have the same language.

but you forget that, after US' indipendence from UK, american scholars claimed
to speak another language, separated from their motherland. This is obviously
due to political reasons, but they actually achieved to change their English
language into a new standard, endowed not only with a different pronunciation,
but also with a different spelling, though not very remarkable (think of -ize
instead of -ise, "harbor" instead of "harbour"). thus, together with their own
army and navy, thery created for themselves another kind of English. anywaym,
today in modern linguistics, as you say, there is no proper difference between
languages and dialects, but only a political one (when the "proper differences"
are not so important, like for French and Breton, or even for Italian and
Sardinian).
for what concerns the debate "lex, law, gesetz..." I think the problem is very
hard to face mainly because we speak about oral cultures, that means that the
concept of written law had presumely its birth after the languages of these
germanic tribes splitted into different branches.

Diego