Spanish or French. Most American students encounter these options in middle school or high school, when their brains are the most receptive to new languages before gradually declining in later life.
Regardless of the neverending complaints students have about tenses, conjugation, and pronunciation, these options make sense. Although English is a Germanic language, it has a significant Romance vocabulary, leading to a decent amount of easily-identifiable and memorizable cognates. Furthermore, Spanish has an estimated 600 million speakers worldwide while French has around 321 million, with both languages being the official language of dozens of countries around the world.
Spanish and French are Romance languages with other well-known languages including Italian, Portuguese, and Romanian. This language family is descended from Vulgar Latin (informal Latin spoken from the Late Roman Republic onward, unlike the Classical Latin used for official writings and government); their designation as “Romance” languages comes from the Vulgar Latin adjective Romanicus (“in the style of the Romans”), as their existence is due to the Romans who spoke and spread Latin throughout most of Europe. Other less well-known Romance languages include Catalan (spoken in Northern Spain and parts of France and Italy), Occitan (the modern name given to numerous dialects spoken in southern France), Sardinian (spoken on the island of Sardinia, an autonomous region of Italy), and Haitian Creole (along with other creole languages). While dozens more languages within this family exist, identifying and quantifying them is a hazy process, as “language” is ambiguously defined (what separates a language from a dialect, basilect, or other language variant) and there is no precise threshold of what extent Latin influences “makes” a romance language.
Nonetheless, the core languages are inarguably similar in heavily Latinate vocabulary, leading to a certain degree of mutual intelligibility. However, it’s important to note that these similarities are not universally constant; for instance, the similarity between Spanish and Portuguese allows speakers to often understand one another whereas French and Romanian speakers may have more difficulty.
“Water” in various Romance languages:
Latin: aqua
Spanish: agua
Portuguese: água
French: eau
Italian: acqua
Romance languages also generally share similar grammar structures. Many notably follow Subject-Verb-Object (SVO) word order. Take the Spanish sentence El gato come una manzana (The cat eats an apple): the subject (“the cat”) does the action (i.e. the verb) of “eating” the object (“the apple”). Furthermore, many share verb conjugations based on establishing tense, making verbs formal or informal, and indicating plurality versus singularity. Finally, most Romance languages have gendered language (masculine and feminine), unlike Latin’s three (masculine, feminine, and neuter), a fact that native English speakers (and I won’t lie, myself included) take a while to come to terms with.