Conscious language learning
Compound and coordinate speakers are claimed to be more proficient in all spheres of learning than persons who are good at one language only. In other words, language mastering is a good ground for mastering all other fields of knowledge.
They say it is associated with the ability to see general notions in more than one way, the ability which is acquired in the course language mastering process.
Nonetheless, it is a no rare thing that learning the first language is stopped or intervened by another one, a speaker (children in particular) may result in being weak in both ones. It is usual practice with immigrant children that don't have full access to the learning of new language and are already out of their mother tongue area. This occurrence is known as semilingualism.
Access to educational resources is of the vital importance. If not taught in the proper way or even constantly discouraged, a child of any origin and potential capabilities may lose all his skills even if he developed some of them up to a certain moment. As a consequence, an adolescent grows up to be no good at none of the languages.
No surprise that to pick up any language to a scientifically-sufficient degree, education needs a state support that must be a part of general state policy sponsored and endorse at an adequate level. Actually, it must be one of the language policy priorities (provided a government takes care of own population of course) aimed at bringing up generations of literate and patriotically-minded citizens.
