Is it a wash trade if I sell an ADR while buying its underlying in a foreign stock market?

Yes.

A wash sale occurs if you sell or trade substantially identical securities or their options within a 30 day period. Keyword here is "substantially identical securities". Pub 550 goes into all the detail you'll find on wash sales here. One example in Pub 550 I think applies here is that the IRS defines debt of the same company that is convertible into common stock as 'substantially identical' to the common stock itself. Since the ADR is technically convertible to the underlying security at any time, then I think this clearly falls under the 'substantially identical' definition.