If i start my career with one martial art and become mature in that, will that particular martial art become my expertise and will other martial arts always be secondary like a native language and second language?
"Wide or deep"? is a classical question in a lot of different domains, and I think the answer depends largely on what your goals are and how you define "expert." You will generally have some set of base techniques–usually but not always from a single style–that you will learn to instinctively fall back on under times of stress, but this doesn't preclude becoming an "expert" in a wide variety of other techniques.
So first ask yourself, "what is my goal"?
Is your goal…
The short answer to your question is that it is absolutely possible to become an expert (depending, again, on how expert is defined) in more than one martial art, but your time is not infinite. So if you have 5 hours a week to spend on martial arts, which will be a more efficient way to reach your goals, to spend 5 hours in one martial art, or 1 hour in five?
When you've been practicing one art for five years and decide to add another one, if you still have 5 hours a week, do you allocate 3 hours a week to the new art and 2 to your old one? How will that impact your training?
Unless your goal specifically requires cross-training–and sometimes even then–I generally would recommend reaching some stable state in one martial art (whether that is an "expert" is up for debate), and then deciding if you want to add another one, why, and how much time you are willing to devote to the matter.
The Samurai were advised to master seven different martial arts. I don't expect to master even one in my lifetime. I've been studying Aikdio for 20 years or so now and Tai Chi for one. I've also studied Shindo muso ryo jodo and mugairyu (both for far too short a time). I'd like to study Bagua.
I'm not sure that I'm comfortable with the assumptions underpinning the question. I'm not comfortable with the notion that "mastery" is objectively observable. (We could discuss the criteria for Shihan, but that would be a discussion outside the scope of SE). I'm not comfortable with the analogy with language - there are plenty of people who are fully conversant in their second language, or their third or their ninth.
Below it all I'm uncomfortable with the goal of "mastery". If you study martial arts to be a master, then go to a black belt factory and lay down the money. If you study martial arts to learn, then learn from each teacher as far as they can take you.
IMHO, it's not only possible, it's advisable.
Here is the reason: martial arts (in general) are divided into Hard and Soft
A hard technique meets force with force; either with a head-on-force blocking technique, or by diagonally cutting the strike with (one's) force. It is an example of the defender using the attacker's force and momentum against him or her. Although hard techniques require greater strength for successful execution, it is the mechanics of the technique that accomplish the defense.
The goal of the soft technique is turning the attacker’s force to his or her disadvantage, with the defender exerting minimal force. With a soft technique, the defender uses the attacker's force and momentum against him or her Here are some examples:
the biggest asset in hard style is force and conditioning
the biggest asset in soft style is technique
I would add one more gradation: semi-hard styles (Wing Chun); the biggest asset there is speed.
Now the fun begins: as far as I know,
all that looks pretty much like Rock-paper-scissors game!
So, in order to be successful, you should have in your toolbox at least 2 instruments out of 3 - having those you will be able to switch between them and properly adjust to the style of your opponent.
Disclaimer: I am aware that a single style may contain all 3 elements (hard/soft/semihard), so you are able to fill your toolbox without going outside of the style; however, I believe that style, specializing in one of these areas, still might give you more