Not good with Russian, but from what I can tell, there's no verb in the sentence https://en.wiktionary.org/wiki/нужна https://en.wiktionary.org/wiki/нужный#Russian "мне" is I think dative--my best take on a literal translation would be "You/ to me/ necessary." Maybe think of it as a distant relative of "Bei Mir Bist Du Schoen" or "You Are So Beautiful to Me", where the sentiment is "I find you beautiful", but because love loves to turn subjects into objects and objects into subjects, presto change-o.
Since the present tense of 'to be' is rarely verbalized (never?) and нужна really being what we would call in English a predicate adjective, it all makes sense. An adjective must follow case rules. Thanks for the German example. Also it reminds me of Spanish where you say "Me gusto la cuidad" to mean 'I like the city' but literally it means 'The city is pleasing to me'. THANKS!
You can translate this as 'you are needed by be' I'm not fluent, but i imagine if you are trying to focus on the 'you' part, you'd have to stress the 'ты' verbally like we would in English or add additional flavour like you would if you were texting this (by either making ТЫ in full caps or adding something like 'and no other' or whatever would be most appropriate in Russian)