Let's say I like new language X that compiles to LLVM or native code. Will it get mobile dev capabilities for free from WASM, or should I target a more established language stack for doing mobile dev like Java.
WebAssembly is a low-level runtime currently targeting the existing web platform. There are no plans to add wasm-specific web platform APIs [0]. You will not gain existing capabilities other than those already exposed to mobile websites. Non-web embeddings will be possible [1], but are not currently being developed.