How do I localize text in SwiftUI? 

SwiftUI localizes string literals for you. Text("Welcome") does not take a String; the literal becomes a LocalizedStringKey, and the view looks that key up in Localizable.xcstrings (or Localizable.strings) for the language the app runs in. If no translation exists, the key itself is shown, which is why an app without translations still looks right.

The same applies to the title parameters of Button("Save"), Label("Message", systemImage: "envelope"), TextField("Email", text: $email), Toggle, Picker and .navigationTitle("Settings"). Add a String Catalog to the project (File > New > File from Template, then String Catalog) and build: Xcode extracts every literal into it, and each later build keeps the catalog in sync with the code.

Variables are the main pitfall. A String value is not localized, because SwiftUI assumes it is user content:

Text(item.name)                     // shown as is
Text(LocalizedStringKey(item.key))  // looked up as a key

Use the second form only for keys you control; nothing is extracted for it at build time, so the entries must already exist in the catalog.

Put dynamic values into the literal with interpolation. Text("Hello, \(name)") looks up the key Hello, %@ and inserts the value, and Text("\(count) items") produces %lld items, which you can turn into a plural rule in the catalog. Do not concatenate pieces with +; translators need the whole sentence.

For text that must never be translated, such as a brand name or a code, use Text(verbatim: "Localizely").

When a string travels through your own types, declare the property as LocalizedStringResource rather than String. It can still be initialized with a literal, Text accepts it, and the string stays localizable.

Add a comment where the literal alone is ambiguous: Text("Book", comment: "Verb: reserve a table"). Xcode exports it to translators together with the string.

To see another language in a preview, set the locale environment of the view: ContentView().environment(\.locale, Locale(identifier: "de")). To run the whole app in a language, use Product > Scheme > Edit Scheme > Run > Options > App Language. For strings created outside views, String(localized: "Welcome") (iOS 15 and later) reads the same catalog.