// Check the preference:
isDark, _ := theme.IsDarkMode() 

// Change the Palette based on the preference:
var palette material.Palette
if isDark {
    palette.Bg = color.NRGBA{A: 255}                         // Black Background
    palette.Fg = color.NRGBA{R: 255, G: 255, B: 255, A: 255} // White Foreground
} else {
    palette.Bg = color.NRGBA{R: 255, G: 255, B: 255, A: 255} // White Background 
    palette.Fg = color.NRGBA{A: 255}                         // Black Foreground
}

// Your dictionary (in that case using x/text/catalog):
cat := catalog.NewBuilder()
cat.SetString(language.English, "Hello World", "Hello World")
cat.SetString(language.Portuguese, "Hello World", "Olá Mundo")
cat.SetString(language.Spanish, "Hello World", "Hola Mundo")
cat.SetString(language.Czech, "Hello World", "Ahoj světe")
cat.SetString(language.French, "Hello World", "Bonjour le monde")

// Get the user preferences:
userLanguage, _ := locale.Language()

// Get the best match based on the preferred language:
userLanguage, _, confidence := cat.Matcher().Match(userLanguage)
if confidence <= language.Low {
    userLanguage = language.English // Switch to the default language, due to low confidence.
}

// Creates the printer with the user language:
printer := message.NewPrinter(userLanguage, message.Catalog(cat))

// Display the text based on the language:
widget.Label{}.Layout(gtx,
    yourTheme.Shaper,
    text.Font{},
    unit.Dp(12),
    printer.Sprintf("Hello World"),
)