It's on my list of things to tackle as part of the Incremental Language Development project.
In the meantime, there is a built-in solution that is fairly fast, which is:
Needs["MachineLearning`"]; ToUpperCaseUnicode[{"éàÇœßþ\[Sigma]\[FinalSigma]", "ijķnjđӽծ", "ÿ"}] {"ÉÀÇŒSSÞ[CapitalSigma][CapitalSigma]", "IJĶNJĐӼԾ", "Ÿ"}
(not sure how to copy-paste the sigma etc without it turning into \[Sigma])