Not nearly as much as one might think.
For instance, I am relatively well versed in the Romance languages. I can read Spanish quite well, but not fluently. I can read a bit of French. And I have studied reading Italian and Portuguese for a bit.
So one would think that with all that Romance under my belt, I could just jump right into some new Romance languages and read them just like that, right?
Not so fast now.
Lately I have been going through lots and lots of Occitan texts on the Net. Occitan is approximately between Spanish and French. Honestly, I can’t make heads or tails of Occitan. Sometimes I can pick out a bit of information that I am looking very hard for, but mostly I just throw up my hands. My online translator calls Occitan “Catalan” and tries to translate it into English. Some say that Catalan and Occitan are one language. According to my translator, that is not so. Running the Catalan translator through Occitan fixes it up a bit, but it still leaves a gigantic steaming mess on the page. It’s nearly useless.
With Portuguese, Spanish and French, one would think Catalan would be a breeze, right? Think again. My translator is almost always able to grab it, but sometimes it can’t. When it can’t, I am stuck with Catalan and I am well and truly lost. Once again, I just throw up my hands. Obviously, it looks like some kind of Iberian language, but it’s so screwed up and crazy that you just don’t want to bother with it.
It’s said that Aragonese is nearly a Spanish dialect. Intelligibility is on the order of 80%. But try reading an Aragonese text sometime. It’s clearly derived from something like Spanish, but it’s so screwed up and crazy that you just want to run away from it. Try to read it and you are quickly lost and angry. My online translator thinks that Aragonese is Spanish. Run Aragonese through the Spanish translator and it fixes it up a bit, but it still a crazy mess and you can’t make a lot of sense of it.
Galician is a sort of Portuguese-Spanish hybrid that is often intelligible to many Spanish speakers. But don’t bother with trying to read Galician texts. They’re a frustrating mess. I dipped into it a bit, but it’s so screwed up and confusing that I quickly gave up.
One would think that with a bit of French under the belt, one could pick up on the various French patois of the langues d’oil. Forget it. It looks like a chaotic disaster on the page. The translator calls the various patois French. Running them through a French translator in general doesn’t really improve matters all that much. It’s still a messy disaster.
The moral to the story is don’t think that semi-getting a few languages under your belt is going to help you even with reading closely related languages. Things are not so simple.