L. Vygotsky, the Mozart of the Psychology of Learning, also wrote:

“A foreign language facilitates mastering the higher forms of the native language. The child learns to see his language as one particular system among many, to view its phenomena under more general categories, and this leads to awareness of his linguistic operations”. 

Not an article, but the following well-known quotation comes to mind (I don't know the original German):

“Whoever is not acquainted with foreign languages knows nothing of their own.”

Johann Wolfgang von Goethe, 1749–1832

L. Vygotsky, the Mozart of the Psychology of Learning, also wrote:

“A foreign language facilitates mastering the higher forms of the native language. The child learns to see his language as one particular system among many, to view its phenomena under more general categories, and this leads to awareness of his linguistic operations”. 

You may read this article. I think it's somehow useful.

http://www.albany.edu/faculty/ikecskes/files/babyloniaproofkecskes.pdf

You may read this book "Línguas em contacto. "Saber sobre" o que as distingue" by Cristina Martins. It doens`t talk exactly about a foreign language but  it may help, because it prove that portuguese young speakers who know a second language improve their skills at native language.  

I certainly remember that when I studied German in High School, I felt that I learned more about English, my native language.

thanks!

This problem, perfectly intuited by L. Vygotski as one or our colleagues has pointed out, has to do with the nature of the discipline called ‘linguistics’. Linguistics is nothing but the description of the verbal behaviour of speakers. The verbal behaviour of speakers is to be examined in three levels. a) It is universal, that is, it can be described for all languages in the world since it is rooted in the nature of human subjects who whenever they speak they will speak in a particular language, that is, German, French, Italian or Greek, etc. b) It is historical, that is, a cultural object made in history with the participation of all speakers in a particular territory, thus constituting a speech community (=a particular language, Spanish, Greek, Swahili, etc.). And finally c) it is individual, since the only one who speaks is the individual subject who alternates with the another one (the I and the You) thus constituting diálogos (=diá-lógos=through the word), the interchange between the I and the You thus referring to their conscience. The individual speaker creates something meaningful (universal level), uses the means of expression of a particular language (Russian, English, Portuguese, etc.) (historical level), to express his meaningful intentional purpose (indivual level). So language is the creation of meanings (universal level); a (particular or historical) language is a set of forms, contents, rules, procedures, attitudes and beliefs in vogue in a particular speech community; and speech is nothing but the meaningful intentional purpose of the individual speaker.

Speakers have a double dimension: they are free, that is, creative, absolute, transcendent, on the one hand and on the other, they are-together-with others, that is, they make themselves with speaking (creative and absolute) and they make themselves in participation with others, their co-speakers, and since they are born at a particular moment of history they make themselves in history, that is, they are historical, that is, limited and contingent.

In this sense speakers live language, that is, they intuit, create, perform, speak and say, use, evaluate and even speak of language. The problem posed in this question has to do with the evaluation by speakers of their speech. Speakers will evaluate their expression whenever they speak. They will correct others and even will correct themselves when they find out that their expression is not either congruent and coherent (universal level); correct, in accordance with the rules in force in the speech community they speak (historical level); adequate to the object they speak of, appropriate to the subject they speak to, and opportune in the moment they speak (individual level). When a speaker learns a foreign language he will immediately evaluate, that is, compare his speech with the kind of speech he is just learning.

This problem has to do as well with the modes of thinking in force in a particular language.

Article International Journal of Language and Linguistics Modes of T...

Book Modes of Thinking, Language and Linguistics

 thank you!!

Prof. Martinez' answer is very profound and insightful, as is Prof. Sideeg's citation of Vygotsky, but the average bilingual or student of a second language is merely focused on its immediate use and the learning task involved, and with most foreign language teaching today dealing only with "communicative competence" (low-level interaction) and not the language as a medium for learning, there is little chance of benefit for the native language. English speakers are in the peculiar position that most higher-order vocabulary is borrowed from French or Latin, so that learning Spanish CAN potentially help with their acquisition of this "Franco-English", as I like to call it. In my own use of Spanish as an academic language, I have learned some words first in Spanish before encountering them in English (cotidiana vs quotidian, for example), but unless these comparisons are explicitly pointed out to students, they will rarely recognize them on their own, as the L2 is usually compartmentalized. It is the rare student who will reflect on the comparison of the L2 and the L1. Even students who have studied German, for example (myself included), are unlikely on their own to recognize the connection between case suffixes in German and prepositions in English (and I have found it very difficult even to get students to recognize this). If someone is introduced to an L2 by a teacher who is linguistically sophisticated (very rare) and the grammatical comparisons are explicitly pointed out (very rare), I can see that there will be some real metalinguistic effect, but most teaching/learning experiences militate against that. It is only the rare introspective individual who will identify cross-linguistic factors and become metalinguistically more sophisticated, with positive effects on the native language. Further, it is difficult to generalize -- someone studying Chinese will have a very different experience from one studying Russian, for example. Quechua speakers view the past as before them, as it can be seen, while the future is behind them, as they move through time, where it cannot be seen, and it flows over them and becomes the past. But this sharp difference in perspective is not likely to come easily to most learners of Quechua as they struggle with the complex morphology of the language, even if it is pointed out.

    Rudy Troike

Thank you all. I should make my question more clear. I wonder if the difference in teaching methods (including the use of metacognitive strategies) and media (ex. the style of the books) of L2 can benefit L1 especially when we have students with learning difficulties.

Most  languages European today  are classified as  Indo-European except Hungarian and Finnish i think within this frame were Latin and Greek which gave birth to most of spoken and written languages in the western hemisphere So to a serious learner  they should be no problem.when crossing the bridge from L1 to l2 if correctly equipped with as Maria quotes Meta-cognitive tools. .   

J'avais une enseignante qui travaillait la-dessus. Tu peux te renseigner sur ses recherches : Harriet JISA

Hi..Please check those results..Some stuff there might be useful, perhaps INDIRECTLY:

https://scholar.google.com.eg/scholar?hl=en&q=learning+foreign+language+improve+native+language+skills&btnG=&as_sdt=1%2C5&as_sdtp=

Pronunciation plays a crucial role in communication, and it can bring about intelligibility problems. It can be stated that foreign language learners should be exposed to the target language not only in written but also orally in order to acquire the sound system correctly. The teacher with mispronunciation is the fundamental reason for miscommunication among the learners.

https://www.google.co.in/url?sa=t&rct=j&q=&esrc=s&source=web&cd=141&ved=0ahUKEwiws_DHpIPLAhWEHpQKHR5DCsY4jAEQFggaMAA&url=http%3A%2F%2Fdergipark.ulakbim.gov.tr%2Fjlls%2Farticle%2Fdownload%2F5000084283%2F5000078380&usg=AFQjCNGPs_bxhSpzdO0CwJ-2EFHQJKqPzQ&sig2=out2r3TzEL1IhmASrJTHiw&bvm=bv.114733917,d.dGo&cad=rja

Age of  exposure to L2 is inversely proportional in acquiring  correct pronunciation. Classes i think are not enough students have to be in continuous contact with the L2 environment in order to be able to think in L2 language.