The key takeaway here is that you can't correctly process a string if you don't what language it's in. That includes variants of the same language with different rules, eg en-US and en-UK or es-MX and es-ES. If you are handling multilingual text the locale is mandatory metadata.
The lowercase of "DON'T FUSS ABOUT FUSSBALL" is "don't fuss about fußball". Unless you're in Switzerland.