So i've been trying to figure this one out for a while. after the Romans lost the peninsula to Goths and then Lombards, where did the northern italian states come from, and their language and culture?.
Remnants of old Latin subjects of rome?
i know that Italy in these ages suffered quite a significant hit to its population, were these Italians living under the germanic kingdoms all along in small number, developing their culture and language? and when and how did the people of this Italian culture start ruling their own city states and the likes
Well, I'm not going to touch the "culture" aspect of this question because, to a certain extent, it's unanswerable.
As far as the "modern Italian language" goes, the answer is the 19th century. The language spoken throughout the Italian peninsula is descended from the Tuscan dialect. While the use of this language for literature was championed in by Dante as early as the 14th century, it was not adopted throughout the region until unification. There are a number of interesting studies on this process, but Martin Clark's, The Italian Risorgimento is a nice introduction to the topic as he effectively weaves together historiography with actual history. The 19th century entry in the Oxford Short History of Italy (Davis ed.) series is good as well.
Like France before them, the proponents of Italian unification believed that a single language was was needed to create a modern nation-state. Indeed, it was not until the French Revolution that the myriad dialects of French were wiped out. In Italy, an even smaller percentage of people spoke "Italian."
From Clark's volume:
There was no argument, in Italy, about which dialect should become the official language: Dante had settled that in the fourteenth century. Every time an Italian picked up a book or a newspaper, he was reminded of his cultural identity. It was a vital common bond among the educated. However, very few people actually spoke Italian - only around 2.5 per cent of the population [emphasis mine], mostly in Tuscany and Rome: the rest spoke local dialects, incomprehensible elsewhere.
Like many of the unification endeavors, the thrust to impose Italian began in the north. Piedmontese agrarian associations banned the use of local dialects and in 1852, the University of Turin made Italian the official language of the institution. Elsewhere in the peninsula, Italian had to be imposed with force.
People living in the Italian peninsula spoke a variety of languages. As stated in the quote above, they were not always easily understood. Zeneize, the dialect of Liguria, is more closely related to French than it is to Italian in terms of grammar. Zeneize is, to my knowledge, still spoken by older people in Genoa, the capital of Liguria and more resistant to unification relative to the other cities of northern Italy.