Literature on that matter is plentiful. Before you browse randomly on the Internet I would suggest you start by having a look at the references on the website of the American Council on the Teaching of Foreign Languages that will give academic references under the following headings:
- Language learning correlates with higher academic achievement on standardized test measures.
- Language learning is beneficial to both monolingual English and English language learners in bilingual and two-way immersion programs.
- There is evidence that language learners transfer skills from one language to another.
- There is a correlation between second language learning and increased linguistic awareness.
- There is a correlation between language learning and students’ ability to hypothesize in science.
- Language learning can benefit all students.
There is a correlation between young children’s second language development and the development of print awareness.
- Heritage learners who use their language skills to interpret and translate for family members experience higher academic performance and greater self-efficacy .
- There is a correlation between language study and higher scores on the SAT and ACT Tests.
- There is a correlation between high school foreign language study and higher academic performance at the college level.
You will find more references on the website of The International Federation of Language Teachers.
I recommend as well those two articles published on The Conversation :
- Keeping actively bilingual makes our brains more efficient at relaying information
- Explainer: how are learning languages and music linked?
This answer was made ignoring the word "intelligence" you use in the title of your question that could be subject to controversy and considering the core of your question is more precise.