How to convert cyrillic into utf16

Building on Jeroen Mostert's helpful comment, the following works robustly, assuming that the input file contains no NUL characters (which is usually a safe assumption for text files):

# Sample value pair; loop over file lines omitted for brevity.
$nline = '1 кириллица'.Split(' ', 2)

$properties = [ordered] @{
  id = $nline[0]
  # Insert aux. NUL characters before the 4-digit hex representations of each
  # code unit, to be removed later.
  value = -join ([uint16[]] [char[]] $nline[1]).ForEach({ "`0{0:x4}" -f $_ })
}

# Convert to JSON, then remove the escaped representations of the aux. NUL chars.,
# resulting in proper JSON escape sequences.
# Note: ... | Out-File ... omitted.
(ConvertTo-Json @($properties)) -replace '\\u0000', '\u'

Output (pipe to ConvertFrom-Json to verify that it works):

[
  {
    "id": "1",
    "value": "\u043a\u0438\u0440\u0438\u043b\u043b\u0438\u0446\u0430"
  }
]

Explanation:

• [uint16[]] [char[]] $nline[1] converts the [char] instances of the strings stored in $nline[1] into the underlying UTF-16 code units (a .NET [char] is an unsigned 16-bit integer encoding a Unicode code point).

  • Note that this works even with Unicode characters that have code points above 0xFFFF, i.e. that are too large to fit into a [uint16]. Such characters outside the so-called BMP (Basic Multilingual Plane), e.g. , are simply represented as pairs of UTF-16 code units, so-called surrogate pairs, which a JSON processor should recognize (ConvertFrom-Json does).
  • However, on Windows such chars. may not render correctly, depending on your console window's font. The safest option is to use Windows Terminal, available in the Microsoft Store

• The call to the .ForEach() array method processes each resulting code unit:

  • "`0{0:x4}" -f $_ uses an expandable string to create a string that starts with a NUL character ("`0"), followed by a 4-digit hex. representation (x4) of the code unit at hand, created via -f, the format operator.

    • This trick of replacing what should ultimately be a verbatim \u prefix temporarily with a NUL character is needed, because a verbatim \ embedded in a string value would invariably be doubled in its JSON representation, given that \ acts the escape character in JSON.

  • The result is something like "<NUL>043a", which ConvertTo-Json transforms as follows, given that it must escape each NUL character as \u0000:

    "\u0000043a"
    

• The result from ConvertTo-Json can then be transformed into the desired escape sequences simply by replacing \u0000 (escaped as \\u0000 for use with the regex-based -replace oeprator) with \u, e.g.:

    "\u0000043a" -replace '\\u0000', '\u' # -> "\u043a", i.e. к
  

There must be a simpler way of doing this, but this could work for you:

$temp = foreach ($line in (Get-Content -Path 'C:\Users\users\Downloads\cyrillic.txt')){
    $nline = $line.Split(' ', 2)
    # output an object straight away so it gets collected in variable $temp
    [PsCustomObject]@{
        id    = $nline[0]   #stores "1" from file
        value = (([system.Text.Encoding]::BigEndianUnicode.GetBytes($nline[1]) | 
                ForEach-Object {'{0:x2}' -f $_ }) -join '' -split '(.{4})' -ne '' | 
                ForEach-Object { '\u{0}' -f $_ }) -join ''
    }
}
($temp | ConvertTo-Json) -replace '\\\\u', '\u' | Out-File 'C:\Users\user\Downloads\data.json'

Simpler using .ToCharArray():

$temp = foreach ($line in (Get-Content -Path 'C:\Users\users\Downloads\cyrillic.txt')){
    $nline = $line.Split(' ', 2)
    # output an object straight away so it gets collected in variable $temp
    [PsCustomObject]@{
        id    = $nline[0]   #stores "1" from file
        value = ($nline[1].ToCharArray() | ForEach-Object {'\u{0:x4}' -f [uint16]$_ }) -join ''
    }
}
($temp | ConvertTo-Json) -replace '\\\\u', '\u' | Out-File 'C:\Users\user\Downloads\data.json'

Value "кириллица" will be converted to \u043a\u0438\u0440\u0438\u043b\u043b\u0438\u0446\u0430

Here's a way simply saving it to a utf16be file and then reading out the bytes, and formatting it, skipping the first 2 bytes, which is the bom (\ufeff). $_ didn't work by itself. Note that there's two utf16 encodings that have different byte orders, big endian and little endian. The range of cyrillic is U+0400..U+04FF. Added -nonewline.

'кириллица' | set-content utf16be.txt -encoding BigEndianUnicode -nonewline
$list = get-content utf16be.txt -Encoding Byte -readcount 2 | 
  % { '\u{0:x2}{1:x2}' -f $_[0],$_[1] } | select -skip 1
-join $list

\u043a\u0438\u0440\u0438\u043b\u043b\u0438\u0446\u0430