Understanding how developers combine programming languages in practice reveals the hidden structure of the software ecosystem: which languages are used as complements, which define coherent technology stacks, and which bridge disparate communities. We present a three-phase empirical pipeline that mines Stack Overflow posts by hundreds of thousands of developers across 186 programming languages, applying FP-Growth frequent itemset mining, Latent Dirichlet Allocation topic modeling, and Louvain community detection on a weighted co-usage graph, with the goal of characterizing co-usage coupling, latent developer specializations, and macro-level ecosystem structure simultaneously from behavioral data. FP-Growth identifies tight coupling clusters such as shell/bash, Swift/Objective-C, and the C-family with lift values far exceeding what individual language popularity predicts. LDA produces 25 developer profiles including Apple-platform developers, scientific and hardware programmers, functional/academic programmers, and two distinct Unix scripting sub-profiles. Louvain partitions the language graph into three macro-communities: web/enterprise, Apple ecosystem, and systems/scientific, and identifies Java as the highest-degree hub connecting all three. All three methods independently converge on the same ecosystem structure, providing strong cross-method validation of the findings.