I think careful "language design" should also extend to the copy on the app itself. Things like tool tips, menu item titles and other types of explainatory text on an interface often don't articulate the feature in a clear way. Sometimes it's too vague or sometimes too technical.

In either case (marketing or UI design) it seems like it's worth it to hire a copywriter to go over the text. We hire graphic design professionals, seems reasonable to do the same for language design.