There are two possibilities: either you sell the publisher worldwide publication rights, or US/North American/English language rights only.
Let’s assume the publisher is a US publisher with no foreign operations.
1. If the author sells worldwide rights, without limiting them to English language, then the publisher will also have the right to commission translations and publish in foreign markets and languages. Any foreign publications and/or translations would be done by a foreign publisher. The US publisher would then receive a royalty from the foreign publisher, and the publishing contract will specify how the author and the US publisher split this money. Customarily, the author would receive 25% to 50% of this royalties paid by the foreign publisher to the US publisher.
2. If the author does not sell worldwide/translation rights, then he will need to identify a translator and/or a foreign publisher. Since the author’s ability to market the book in a foreign language he presumably does not master, it is best to find a foreign publisher to secure a qualified translator, shoulder the cost of the translation and market the translated book. This is where an agent provides invaluable support, even to self-published authors. Any contract with a foreign publisher would then be a publishing contract for a specific language. For ebooks, it is customary to sell worldwide rights for a specific language which enables the foreign publisher to reach potential readers outside the main geographical area for that language.
In short, the time to worry about translation rights are when signing the original publishing deal. This is the time to decide whether to delegate all marketing of the book, including in foreign languages, to the US publisher, knowing the royalties perceived from translation will be less than if finding foreign publishers alone, or to trust that the book will be successful enough to convince foreign publishers to translate it.
In fact, if the book is indeed successful, in English, finding an agent to deal with foreign publisher should not be too difficult. An agent typically takes 15% of the royalties as payment, rather less than the 50 to 75% usually required by the original language publisher. If the book is not successful, the original publisher is unlikely to secure translations deals anyway.
In other words, the question is summarized in one sentence, if the book turns to be a best-seller, does the author want to keep 85% of the royalties from translation or as little as 25%?
That is the only relevant question to ask oneself when signing a publishing deal in the original language.
For more info on copyrights issues : Literary Law Guide For Authors