It's necessary because of "in". It's essentially saying "in the Dutch (language)"
But as MariekeGro pointed out, the other way of saying this is "Ik probeerde Nederlands te praten" (which is literally translated as "I tried to talk Dutch" - not correct English as it should be "I tried to talk in Dutch"). I do have to wonder why they used "praten" here instead of "spreken". In this sort of context, I prefer "to speak" in English and learned this with "spreken" in other language study sources.
To me this looked like it meant "I tried to speak in the Netherlands", suggesting that the speaker was having difficulty speaking while in the Netherlands. Although now that I think of it, if Nederlands was a reference to the country, there would be a "de" in front of it, because the Netherlands is plural. Am I correct, in reading the name of the country as "the low lands" or some such? (Large parts of Eric Flints 1632 saga take place in and around the Low Countries (which include the Netherlands, Belgium, Luxembourg, etc).
I'd say that most (if not all) countries don't get an article in Dutch unless they're plural, the article is part of the country's name, or they're a common noun (het Verenigd Koninkrijk). 'I tried to speak in the Netherlands' would be 'Ik probeerde in Nederland te praten' ('Nederland' is singular in Dutch).