Does having developed a local (La)TeX system help in graduate admission? Suppose that a person was born in country A and the language of country A was his mother language (non-English); he then immigrates to country B (English speaking), is a citizen of country B and has done his undergraduate entirely in country B. This person had no need to typeset anything in his own native language but devoted a lot of time and developed an open-source (La)TeX system for country A; the localized (TeX)LaTeX system that he has developed is now extensively used in country A in academia for typesetting books, theses , ... . He is also a well-known person in country A for his TeX work. His TeX work for country A was very significant that improved the quality of the academic publishing in country A a lot and academics in country A can even write a letter to confirm this.

Now if this person applies for graduate school in electrical engineering (USA), does his TeX work for country A help him in his graduate admission? is there any need to mention his TeX work or will not the admission committee really care?

Note: USA is neither country A nor country B
Like all "extracurricular" activities, it will help, but maybe not as much as you would like. I agree with Jeff that it's an "accomplishment that shows a lot of desirable traits", but fundamentally, if the rest of your application is not sound, this is not going to win them over.

I think it is best to think of this as a tie-breaker - if the rest of your academic achievements are as good as the ones of the next best candidate, this is going to be an unique characteristic that will make you stand out and be more memorable. However, if other candidates have better letters, more research experience, or vastly better grades, the fact that you did something very useful in a fundamentally different area is not going to be valued highly enough.