In[31]:= ToUpperCase@{"éàÇœßþσς", "ijķnjđӽծ", "ÿ"}
Out[31]= {"ÉÀÇŒSSÞ∑∑", "ijķnjđӽծ", "Y"}

Needs["JLink`"];
ClearAll[toUpperCase];
toUpperCase[s_String] :=
    JavaBlock[JavaNew["java.lang.String", s]@toUpperCase[]];

unicodeData = StringSplit[#, ";"] & /@
   StringSplit[Import["ftp://ftp.unicode.org/Public/UNIDATA/UnicodeData.txt"], "n"];
upperCaseData = (FromDigits[#, 16] & /@ # &) /@
  Select[unicodeData, (Length[#] > 12) && (StringLength[#[[13]]] > 0) &][[;; , {1, 13}]];

unichar[s_] := FromCharacterCode[FromDigits[s, 16], "Unicode"];
upperCaseChar[i_] := Module[{r},
  r = Select[upperCaseData, #[[1]] == i &];
  Return[If[Length[r] > 0, FromCharacterCode[r[[1, 2]]], FromCharacterCode[i]]];
  ]
upperCase[s_] := StringJoin[upperCaseChar /@ ToCharacterCode[s, "Unicode"]];

In[127]:= upperCase["foéàçÿœÆijķnjđӽծÿ"]
Out[127]= "FOÉÀÇŸŒÆIJĶNJĐӼԾŸ"

Needs["MachineLearning`"];
ToUpperCaseUnicode[{"éàÇœßþσς", "ijķnjđӽծ", "ÿ"}]