definite article

Using the 'el/la' depends whether we are referring to things 'in general' or not.  

In the sentence you give, these three things are being referred to 'in general' and we always use the definite articles 'el/los/la/las' when we are talking about something in general in Spanish. As you probably know, this is often different to how we might use them in English. For instance:

Los idiomas son interesantes = Languages are interesting

Los elefantes son grandes = Elephants are big

La vida es un regalo = Life is a gift

So, because this sentence is mentioning these three things in the general sense, the pronouns are necessary. 

However, if you are not referring to these things in the general sense, the articles are not necessary. For instance, these sentences now refer to 'specific' things:  

¡Ese elefante es inmenso! = That elephant is huge! (not elephants in general, but that one particularly) 

Algunos idiomas son fáciles de aprender = Some (not all) languages are easy to learn

Su vida fue difícil = His life was difficult

So, looking at your original sentence again, if we were talking to a specific person we could say:

María, mejorarás tu memoria y confianza al aprender un idioma = Maria, you will improve your memory and confidence by learning a language.

In this new sentence these same things are not general anymore, they refer to  'specific' things, and in this case the 'el/la' are replaced by the possessive pronoun 'tu' and the indefinite article 'un'

Hope this helps. Saludos