I think speaking a different language simply for the sake of keeping someone from understanding you is rather pointless. Any language can, in the end, be learnt and there is no means of keeping it confidential unless you do not speak it in public, which, again, shows how pointless it is to speak it for that sake only.
For instance, when I traveled doing some research, my first language as well as English was the best way to communicate for my third language in no way resembled the ones spoken in a different province. However after a month (Even though I could not pronounce any of the words to safe my life) I could already understand and follow most of their conversations simply being exposed to that language continually.
I think the use of any language is simply for comfort sake and perhaps a small influence from inheritance or culture as well. Because you are more comfortable with it you would rather speak your mother tongue than a second language would you not? It is also true that some people learn new languages simply for the fun or interest of it but I think it is more commonly because of a need for understanding others. A second language I believe is usually there for the sake of easier communication as you would try and speak the language known and understood by the most amount of people.
We have 11 official languages here in SA. Even so the majority speaks only one or both of the two most widely spread ones (One of these being English).
There are also “region orientated’ languages. For instance along the South and Eastern coast (where I live) there is a third language spoken that is very common. Many people speak this as their home language along with English as a second language. If you move towards the West coast there are two different varieties of yet a different common language found there. The one spoken more to the South of this region differs considerably from the one used more towards the North. Then when you move inland there is yet another language found that is commonly spoken there.
Thus depending on where you are from you speak two of any of these 6 languages (Yes it is common here to speak two languages and many people speak three or more). Then there is also a large variety of less well known languages spoken in smaller areas. Due to this I feel the language you speak is influenced more by the area you originate from than anything else.
This is why I like the idea of basing the languages according to the followers of Laanx and Talad. There different people stayed separated from each other for many generations and it would only be natural for them to evolve their languages differently?
I am sure even those that lived on their own and had no religion would have formed a different language.
I do not think however that the languages should be based on ones religion; this should simply be its origin. How well you speak a language would depend on your race and religion. Thus each language would be race specific although quite a few races would be able to speak the same language. If you are of a religion that is opposite to the one your race originated from you would find it easier to understand that language than others even though you might not be fluent in it from the start.
Everyone will still be able to speak the common tongue though and there would have to be an option if you wished to switch over to another language. Either way this will probably be very hard to implement but it would be a lot of fun to have.