Polyglot programming is a term coined by Neal Ford in 2006, and expresses the idea that computer applications should be written in a mix of different programming languages, in order to take advantage of the fact that different languages are suitable for tackling different problems.
Relational databases, for example, are good at enforcing relationships that exist between various data tables.
To discover a relationship or to find data from different tables that belong to the same object, an SQL join operation can be used.
Instead, a NoSQL document database might be used to store unstructured data for that particular part of the problem.
There will still be large amounts of it managed in relational stores, but increasingly we'll be first asking how we want to manipulate the data and only then figuring out what technology is the best bet for it.