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Responding to Jon Udell's infoworld column Tools for Rules, Phil Windley writes about programming using rules engines:

XSLT is probably the first declarative programming language to
go mainstream. Prolog and others were fun, but never caught
on in the commercial environment.

Depending on your definition of "mainstream", here are a couple of earlier examples of programming in a somewhat declarative mode, at least with respect to implicit sequencing: on Unix, awk was one of the first languages that had this feeling. The sequencing of the code depended very much on the input, and was not very explicit in the awk code. In "the commercial environment", RPG was quite similar in this way to awk.