In a hypothetical scenario where a substance can rewire the user's brain to instantly speak and understand another language without the use of nanobots or other technological aspects, the substance would need to interact with the brain in several ways:
Enhance neural plasticity: The substance would need to increase the brain's ability to create new connections and modify existing ones rapidly. This could be achieved by upregulating specific neurochemicals, such as brain-derived neurotrophic factor (BDNF), which promotes the growth of new neurons and the formation of new synapses.
Target language-specific brain regions: The substance would need to specifically target the language-related regions of the brain, such as Broca's area (responsible for speech production) and Wernicke's area (responsible for language comprehension). It would also need to affect the connections between these regions and other areas involved in language processing, such as the angular gyrus and the arcuate fasciculus.
Encode linguistic knowledge: The substance would somehow need to encode the grammatical structures, vocabulary, and phonetic patterns of the target language into the user's brain. This could involve creating or modifying specific neural pathways and connections to store the new information.
Integrate new language with existing language skills: To allow the user to still perceive the substance's language as their own, the substance would need to seamlessly integrate the new language with the user's existing language skills. This could involve creating new connections between the newly formed neural pathways and the user's existing language-related brain regions.
Adapt to individual differences: Since individuals have different language backgrounds and cognitive abilities, the substance would need to adapt its effects to accommodate these differences. This could involve tailoring the substance's neural effects to the specific language background and cognitive profile of the user.
Given the complex and intricate nature of the brain, achieving such effects without the use of nanobots or other technological aspects is highly improbable. Even if a substance could achieve these effects, it would still be challenging to ensure mutual communication between users, as language is a complex and nuanced system that relies on context, culture, and individual experiences. Furthermore, the long-term effects of such a substance on the brain are unknown and could potentially be harmful. As has already been said here, even if you have the knowledge of what sounds to make, that doesn't give you the ability to make these sounds. A better bet would be technology-based, or to bypass the need to rewire the brain completely and has an alien parasite do the translating for you, but that has already been alluded to by Mathaddict in another answer.