Yes, that's true that having an explicit "Error" or "Success" makes message clearer, but i would like to replace it with success written in my langauge. Thats why i asked this question ;)

it's as bit as if I had firmly identified a X-Y problem in the first place. I haven't - at least not firmly. But you're clearly hunting for a solution to a different problem.
In your case: Just switch Liferay's UI from english (default) to the language of your choice (which is it?)
You can do that user by user, and provide a default for the whole installation or a separate one for each site.

It is not really X-Y problem, i've just suggested that it should have been written in my language, but for me the best solution is to remove this word "Success|Error" completely and i really want to achieve it. Can you give me some hints to make it? 

which is it: "remove completely" as you write here, or "replace it with success written in my language"? Because the solution will be different:
The most maintainable way to eliminate it (as it's not a threat if it's contained in the DOM is after looking at the DOM:

Just add CSS to your theme, like (untested)

.alert .lead { display: none; } 

(and you can even use the appropriate aria stuff to make it screenreader-accessible while not shown graphically)

The most maintainable way to display it in your language is to switch to your langauge.