Extracting Foreign-Language Characters

As noted in my first comment below, Excel 2021's UNIQUE function is not case-sensitive. An equivalent using REDUCE, LAMBDA, and VSTACK is case-sensitive, but those functions currently require Excel 365.
My Excel Toolbox now includes the following function to return unique rows or columns from a cell range (contiguous) or array constant or result of an array function:
    =UniquePlus(RangeArray, [LeftRight], [ExactlyOnce], [HasHeader],
        [CaseSensitive])
The first three parameters match Excel's UNIQUE function.
When HasHeader is TRUE, the first row (LeftRight=FALSE) or first column (LeftRight=TRUE) is returned even if it is not unique.
Case-sensitive comparisons are done when CaseSensitive is TRUE.
This function does not require Excel 2021 or newer.
See https://sites.google.com/view/MyExcelToolbox

The Tip's ForeignChars function returns an unsorted string of characters; for example
    ëÄäéÉË
The result will be case-sensitive as illustrated in that example if the function is in a module with the default
    Option Compare Binary
But if the module begins with
    Option Compare Text
the result will not be case-sensitive and the previous example would simply return
    ëÄé
The following version returns a sorted case-sensitive comma-separated result for that example like this
    Ä, É, Ë, ä, é, ë
regardless of Option Compare:

Function ForeignChars2(ByVal Target As Range) As String
'ASCII is 0 to 127; non-ASCII is "foreign"
    Dim Chars As New Collection, char As String
    Dim cell As Range, n As Integer, k As Integer
    Const COMP As Integer = vbBinaryCompare 'case-sensitive
    If Target.Cells.Count > 1 Then
        Set Target = Application.Intersect(Target.Parent.UsedRange, Target)
    End If
    For Each cell In Target
        If WorksheetFunction.IsText(cell) Then
            For n = 1 To Len(cell)
                char = Mid(cell, n, 1)
                If AscW(char) > 127 Then 'non-ASCII
                    If Chars.Count = 0 Then
                        Chars.Add char 'add first
                    ElseIf StrComp(char, Chars(Chars.Count), COMP) > 0 Then
                        Chars.Add char 'add char > Chars(Chars.Count)
                    Else
                        For k = 1 To Chars.Count
                            Select Case StrComp(char, Chars(k), COMP)
                            Case 0 'already added char = Chars(k)
                                Exit For
                            Case Is < 0 'add char < Chars(k)
                                Chars.Add char, , k 'before Chars(k)
                                Exit For
                            End Select
                        Next k
                    End If
                End If
            Next n
        End If
    Next cell
    If Chars.Count = 0 Then ForeignChars2 = "none": Exit Function
    ForeignChars2 = Chars(1)
    For k = 2 To Chars.Count
        ForeignChars2 = ForeignChars2 & ", " & Chars(k)
    Next k
End Function

The Tip's first formula is very clever:
    =LET(wl, A:A, cwl, TEXTJOIN(, TRUE, wl), cl, MID(cwl, SEQUENCE(LEN(cwl)) ,1), SORT(UNIQUE(FILTER( cl, CODE(cl)>127))))
Here are some nit-picking comments:
1. TEXTJOIN(, TRUE, wl) could be replaced by CONCAT(wl) or CONCAT(A:A) for the same result. Using the latter means the wl parameter is unnecessary:
    =LET(cwl, CONCAT(A:A), cl, MID(cwl, SEQUENCE(LEN(cwl)), 1), SORT(UNIQUE(FILTER( cl, CODE(cl)>127))))
2. The Tip's formula returns a vertical array with N rows and 1 column, where N is the number of unique "foreign-language" (non-ASCII) characters. Lucie might prefer a comma separated list in a single cell:
    =LET(cwl, CONCAT(A:A), cl, MID(cwl, SEQUENCE(LEN(cwl)) ,1), TEXTJOIN(", ", TRUE, SORT(UNIQUE(FILTER(cl, CODE(cl)>127)))))
3. Unless specifically stated, most Excel functions are not case-sensitive when applied to text. Neither is the UNIQUE function; therefore,
    UNIQUE({"a";"A";"B";"b"}) returns {"a";"B"}
a mixed-case array. Lucie might prefer all lower-case characters for uniform appearance:
    =LET(cwl, CONCAT(A:A), cl, MID(cwl, SEQUENCE(LEN(cwl)) ,1), TEXTJOIN(", ", TRUE, SORT(UNIQUE(LOWER(FILTER(cl, CODE(cl)>127))))))
4. Here is the same formula with the UNIQUE(...) part isolated for further discussion:
    =LET(cwl, CONCAT(A:A), cl, MID(cwl, SEQUENCE(LEN(cwl)),1), uniq, UNIQUE(LOWER(FILTER(cl, CODE(cl)>127))), TEXTJOIN(", ", TRUE, SORT(uniq)))
5. Lucie might want to list both upper-case and lower-case characters if both appear in the names. The following article describes a case-sensitive equivalent of Excel's UNIQUE function:
https://exceljet.net/formulas/unique-values-case-sensitive
This version of the previous formula includes each unique case-sensitive "foreign-language" character:
    =LET(cwl, CONCAT(A:A), cl, MID(cwl, SEQUENCE(LEN(cwl)), 1), uniq, REDUCE(, FILTER(cl, CODE(cl)>127), LAMBDA(a, v, IF(SUM(--EXACT(a, v)), a, VSTACK(a, v)))), TEXTJOIN(", ", TRUE, SORT(uniq)))
6. Like UNIQUE, the SORT function is not case-sensitive. But the SortPlus function in My Excel Toolbox has a case-sensitive option:
    =LET(cwl, CONCAT(A:A), cl, MID(cwl, SEQUENCE(LEN(cwl)), 1), uniq, REDUCE(, FILTER(cl, CODE(cl)>127), LAMBDA(a, v, IF(SUM(--EXACT(a, v)), a, VSTACK(a, v)))), TEXTJOIN(", ", TRUE, SortPlus(uniq, , , , , TRUE)))
7. SortPlus also has an option for ANSI/ASCII/Unicode order instead of Excel order; that option sorts upper-case before lower-case:
    =LET(cwl, CONCAT(A:A), cl, MID(cwl, SEQUENCE(LEN(cwl)), 1), uniq, REDUCE(, FILTER(cl, CODE(cl)>127), LAMBDA(a, v, IF(SUM(--EXACT(a, v)), a, VSTACK(a, v)))), TEXTJOIN(", ", TRUE, SortPlus(uniq, , , , , TRUE, TRUE)))
Each of these formulas can be copy/pasted into a worksheet for testing.
For more on SortPlus, see https://excelribbon.tips.net/T012575
and https://sites.google.com/view/MyExcelToolbox