Allow individual ValueObjects to pick their preferred display language

Most of the time, we can use context information just fine to choose a language (i.e. the language of the frame that the root object was defined in, if any); but in some cases, synthetic children may be fabricated as root frame-less entities, and then we wouldn't know any better

This patch allows (internal) synthetic child providers to set a display language on the children they generate, should they so choose

llvm-svn: 226634