It is called "code-switching". It happens involuntarily also.

It can be a type of code-switching called "tag-switching" as well.

More details: http://en.wikipedia.org/wiki/Code-switching

Also, I found studies about this topic when I searched further.

The formal term is code-switching, and the specific sub-category is tag-switching for switching words or phrases. It happens involuntarily also.

Tag-switching is the switching of either a tag phrase or a word, or both, from one language to another, (common in intra-sentential switches). In Spanish-English switching one could say, "Él es de México y así los criaron a ellos, you know." ("He's from Mexico, and they raise them like that, you know.")
- Code-switching/Wikipedia