It is nothing uncommon for programming languages to appear, evolve, and go eventually extinct. Some on them are just trendy, others are «working horses» of software engineering. A decade and more ago, multi-language projects and specialists were one in a hundred. By now, things have drastically changed. And keep on changing.
Actually, state-of-the-art programming is hardly possible without harnessing several frameworks at a time. And it is getting hardly possible without using two or more programming languages accompanied with aforementioned frameworks. This change leads us to an idea of polyglot programming which is much more than just having profound knowledge of two, three, or more languages.
Polyglot programming strongly relates to architecture decisions and architecture rationale (see ISO/IEC/IEEE Std. 42010:2011). In this paradigm, software engineers make careful choice of languages, frameworks, tools, and etc. And usually they do know these languages, frameworks, and tools well.