The Spanish constitution, the Constitutional Court, and the Catalan referendum

Secession is a phenomeon and a challenge that some Western European democracies are forced to face. Recently, after a long period of uncertainty the majority of the Scottish population eventually decided in a referendum to stay within the United Kingdom. In Spain, claims of Catalonia to hold a corresponding referendum have so far been rejected. Why, when and how did the Catalanist movement turn towards independence? Is the language issue still paramount, or is it all about money, or about the right to decide? And is this different from national self-determination? Is immigration an obstacle? What alternative forms of accommodation could be considered? Is there a lawful way to independence? How is Europe, how is Germany treating the issue? Would an independent Catalonia be viable, and how democratic would it be?