Use when designing how multiple MPS languages relate or compose — choosing between language referencing, extension, reuse, and embedding, or deciding where a cross-language dependency should live and how generated code interacts. Covers the 2×2 modularity taxonomy adapted to MPS, a decision procedure, per-kind recipes (references + search scopes; subconcepting + overloaded operations; abstract hooks + adapter languages + separated/interwoven generation; guest + composition languages + type bridges), the annotations escape hatch, and per-kind validation. This is the design tier above the aspect-implementation skills: it picks the strategy, then routes into the relevant mps-aspect-* skills (structure, constraints, typesystem, generator, generation-plan, intentions) and mps-tests. Reach for it before authoring aspects, or whenever one language must see, extend, embed, or adapt another.