Well, lets take myself as a study, I grew up in a English / Afrikaans house hold, with more emphasis on English,
I went to English schools.
When I think, its in English, when I speak its in English, however if I am with the Afrikaans family I think in Afrikaans.
Now, I have asked English only friends the following question, when doing Afrikaans in school do you think in Afrikaans or English, most people would Think in English, then Translate what they where thinking in their head and then speak in Afrikaans.
Now think about the following, When thinking in different languages whole different mindsets evolve, different ways of seeing the world, because culture influences language, and each language describes the world in different ways (for example, we have one word for snow, it is snow, Inuit people (Eskimos) have many ways of saying the word for snow, each describing the different types of snow, each meaning a very different thing to them, from our English view point snow is snow and nothing else, not matter if its hard, soft, new, old, etc...
another example, if any one has ever "torn you a new one" in Afrikaans its an experience vs some one crapping on you in English...
So if your child from an early age is exposed to different languages like this on a daily basis it is only going to further brain development, you will probably end up in a situation in the begging where the child is mixing the languages, when communicating but eventually the child will learn to speak/think/articulate fluently in all of them.
Remember their brains are forming and language is developing based on the input the child is getting, so if you show them and apple and they have learnt its an apple, in all the languages that is how it will be wired in their heads vs learning the languages at a later stage where you need to train your mind. plus in this day and age in its good to be multi lingual, also German and Afrikaans are similar in nature.
I would say its good for brain development.