Short Answer
Hume's brief section is about how ideas have regular structure. Your passage mentions languages, but the reading should be broader. Some might restrict themselves in reading this passage in virtue of the structure of concepts. A classic example is that all languages of the world have subjects, objects, and verbs, even if the order is not the same between languages. For instance, English is SVO whereas German is SOV (given the accumulation of compound verbs at the end of the sentence). But a careful reading shows that the section also includes references to the senses, memory, inference, etc.
Long Answer
Hume Predates Modern Linguistics
Hume was born long before Ferdinand de Saussure and modern linguistics. Linguists had engaged in comparative philology noting similarities between Greek, Latin, German, English, Sanskrit, and so on. But the philosophy of language in the modern sense didn't really begin until the 19th century. Hume would have likely been familiar with what would be considered pseudoscientific speculations about languages. But this section is about the relationship about ideas more than language. He says:
I do not find that any philosopher has attempted to enumerate or class all the principles of association... there appear to be only three principles of connexion among ideas, namely Resemblance, Contiguity in time or place, and Cause or Effect. (emphasis mine)
Ideas Are Broader than Concepts and Languages
The section attempts to characterize thought through broad patterns of ideas. He says this at the open of the section:
It is evident that there is a principle of connexion between the different thoughts or ideas of the mind, and that, in their appearance to the memory or imagination, they introduce each other with a certain degree of method and regularity.
This is where the modern notions of morpheme and morphology comes in handy:
In linguistics, morphology (/mɔːrˈfɒlədʒi/) is the study of words, how they are formed, and their relationship to other words in the same language. It analyzes the structure of words and parts of words such as stems, root words, prefixes, and suffixes. Morphology also looks at parts of speech, intonation and stress, and the ways context can change a word's pronunciation and meaning.
But also, today we have a vocabulary to distinguish visualization, auditory experience, etc. from language use and concepts. Hume is concerned in this section with thought writ large.
Thought Has Structures on Many Levels, But Human Thought Has an Essence
But if you read and reread Section 3, one gets a distinct picture that where Hume really dwells is the idea that human thought, while characterized by linguistic structure, is really what might be considered a structured stream of consciousness today. From the section:
A picture naturally leads our thoughts to the original; the mention of one apartment in a building naturally introduces an enquiry or discourse concerning the others: and if we think of a wound, we can scarcely forbear reflecting on the pain which follows it.
Conclusion
While the passage you point out might recognize that languages might have linguistic similarities due to common ancestry, such as languages of the PIE family, or even more broadly that humans may have a universal grammar, the important thing to reflect on is that the passage you cite supports the broader aim of the chapter, which is to argue that human thought has an essence, and that includes structure not only of language and concept, but of the connections between ideas, which more broadly covers qualia and the sensory experiences themselves also. When taken together, both concepts and percepts, patterns emerge, what Hume calls the Association of Ideas.
