However, while all this linguistic diversity is fascinating to me, it's also frustrating. For two reasons. Firstly, I like languages, and it makes me sad, somehow, to see previously vibrant languages drop sharply off the edge on the way to extinction. It's especially frustrating to realize that this was deliberately done in most cases; Franco persecuted any language in Spain other than Spanish, and only the stubborness of the Catalonians and Valencians kept their language viable. Closely related Occitan, in southern France, was more widely spoken than French at the beginning of the 20th century; yet nowadays, it's a language in decline, and Herculanean efforts are being made to revive it. Linguistic defibrilation, as it were.
Secondly, it's frustrating to me because I like to put things into neat categories, and linguistics is a science that resists that trend. There is a lot of argument based on the fact that there is no good and universally agreed upon definition for when a dialect is called a separate language or not. And while linguists often try to divide languages along genetic lines, that also remains difficult because areal and contact features can have as big an influence as genetic features, and it's often very difficult to determine which is which, especially among languages that have been close to each other for a long time.
So, there are lots of arguments among linguists and others as to whether Occitan actually exists as a language, or if it's better classified as the closely related languedoc family, with Gascon, Provençal, Limousin, etc. as independent languages. As an example. Although less controversial, there are always also discussions about whether Catalan and Occitan should be united, since the differences between them are more political than linguistic. And although all linguists recognize this extremely close relationship between Occitan and Catalan, traditionally Occitan is grouped with the Gallo-Italian languages, while Catalan is grouped with the Gallo-Iberian languages. Probably due to political and social realities moreso than linguistic ones; certainly Catalan has had a lot of areal influence from Spanish and Occitan from French, but not nearly enough to disguise the extremely close relationship between the two of them. For that matter, is Valencian really a separate language from the Catalan spoken in Barcelona, or is that just an affectation of the Valencians? What about dialects like Ribagorçan--is that a Catalan dialect that's absorbed some features from the more distantly related Aragonese language, or is it an Aragonese dialect that's absorbed features from Catalan? (The attached picture is from a town in Ribagorça (Benabarre - Spanish, Benabarri - Aragonese, Benavarri - Catalan), by the way. If it wasn't for that crane in the foreground, it'd be a great picture of almost medieval quality.)The situation is a bit worse in Italy, where the word dialetto doesn't really mean dialect, even though it's obviously the same word. Semantic shifts have turned the word into one where it means "regional language". In Italy, only Italian itself has official recognization at a national level, which naturally hastens the absorbtion of languages like Venetian, Piedmontese, Lombard, Sardinian, Sicilian, etc. Some of those are not particularly closely related to Italian except by geography; Venetian, Piedmontese and Lombard, for example, belong to the Gallo-italic, or Padanian group, which is more closely related to Occitan and Catalan than to Italian. Sardinian is not considered to be closely related to any other romance language, except by contact. Conversely, Corsican, on the other hand, is clearly a dialect of Tuscan, which was the basis for standard Italian (although it's been heavily attacked by French in recent decades.)
So trying to pin relationships down between languages and dialects is often fluid, and often more motivated by social or political forces than it is by linguistic ones anyway. And this is probably where linguistics can become interesting to setting designers and GMs. I doubt most gamers are as interested in linguistics for its own sake as I am, but political and social movements can almost always make an interesting backdrop for a game. I like to insert little stuff like that; even if it ends up being little more than campaign color, having a group that proudly refuses to stop speaking a dying minority language, or pushes for separatism, or something like that can also became major campaign elements. If a province erupts in social or ethnic based open revolt, for example... that could be a fascinating thing to have occur in the backdrop of your campaign.