The exact application method varies by patch version. Generally, you'll extract the patch files and either replace existing files or run a patching utility.
The Hydrogenaudio Wiki, which serves as the official knowledge base for foobar2000, explicitly states that by the core application. The developer has no plans to implement an official multilingual interface or provide alternate language versions. This isn't an oversight—it's an intentional design decision rooted in foobar2000's philosophy of simplicity, efficiency, and focusing on core audio functionality rather than interface frills. foobar2000 language pack