"It does not make sense for a contract between two Swiss companies to be governed by a foreign law."
When I read that sentence the "a" before "foreign law" jumped out at me. I feel that the sentence would sound more natural without it. On the other hand, I think one could argue that "a foreign law" means the law of one country, while "foreign law" could refer to the laws of several countries.
What do you think?
Without the article, it means the legal systems of other countries. With the article, it suggests one specific law of one foreign country.