Hire a translator. The truly ambitious self-publisher (with a large budget) can self-publish her book in different languages with the help of a translator. If you intend to publishing in multiple languages you'll need a translator who can translate in multiple languages, or a translator for each language, which can be a significant expense.
Hire proofreaders in each language. If you don't speak the language yourself you'll need a system of checks and balances to be sure the translation is correct and clear.
Distribute your book. Even if you've done everything right up to this point, distribution can prove difficult. It's hard enough for a self-publishing author to get into distribution in English-speaking countries. Distribution in foreign countries with other languages requires an understanding of how the publishing industry operates in each country you've chosen. It's not impossible, but there is an easier and more cost-effective way.
Build up a sales track record in your own language first. Foreign-rights sales can be a nice extra revenue stream, but unproven books tend to be a hard sell. If you choose to publish your book with traditional publishers, foreign rights may be one of the rights they buy, though they may or may not use those rights. Try to hold on to them and sell them yourself through an agent if you can. (You'll make more money and enhance the odds of your book's actually being published in other languages.) If you're self-publishing your book, focus on sales in your own country and other English-speaking countries where you can get into distribution, such as the UK and Canada, before trying to sell foreign rights.
Find an agent who represents your genre and is familiar with foreign-rights sales. Some agents primarily focus on sales to traditional publishers in the United States. Agentquery.com is one of the best resources for finding a good agent.
Work with your agent to find the best foreign publishers and sell the rights in as many countries as possible for wider distribution and better revenue.