The cyrllic string my app receives uses(I believe) the table below: 
said I believe, because all the chars I tested fit this table.
Question: How do I convert such thing to a string, which is unicode by default in my delphi? Or better yet: Is there a ready-to-use converter in delphi or should I write one?
If you are using Delphi 2009 or later, this is done automatically:
type CyrillicString = type AnsiString(1251); procedure TForm1.FormCreate(Sender: TObject); var UnicodeStr: string; CyrillicStr: CyrillicString; begin UnicodeStr := 'This is a test.'; // Unicode string CyrillicStr := UnicodeStr; // ...converted to 1251 CyrillicStr := 'This is a test.'; // Cryllic string UnicodeStr := CyrillicStr; // ...converted to Unicode end; 
var UnicodeStr: string; CryllicStr: CryllicString; begin SetLength(CryllicStr, 1); CryllicStr[1] := char(255); UnicodeStr := CryllicStr; ShowMessage(UnicodeStr); Or, even stranger-looking: CryllicStr := 'ÿ';. Both approaches will display a я character.
First of all I recommend you read Marco Cantù's whitepaper on Unicode in Delphi. I am also assuming from your question (and previous questions), that you are using a Unicode version of Delphi, i.e. D2009 or later.
You can first of all define an AnsiString with codepage 1251 to match your input data.
type CyrillicString = type Ansistring(1251); This is an important step. It says that any data contained inside a variable of this type is to be interpreted as having been encoded using the 1251 codepage. This allows Delphi to perform correct conversions to other string types, as we will see later.
Next copy your input data into a string of this variable.
function GetCyrillicString(const Input: array of Byte): CyrillicString; begin SetLength(Result, Length(Input)); if Length(Result)>0 then Move(Input[0], Result[1], Length(Input)); end; Of course, there may be other, more convenient ways to get the data in. Perhaps it comes from a stream. Whatever the case, make sure you do it with something equivalent to a memory copy so that you don't invoke code page conversions and thus lose the 1251 encoding.
Finally you can simply assign a CyrillicString to a plain Unicode string variable and the Delphi runtime performs the necessary conversion automatically.
function ConvertCyrillicToUnicode(const Input: array of Byte): string; begin Result := GetCyrillicString(Input); end; The runtime is able to perform this conversion because you specified the codepage when defining CyrillicString and because string maps to UnicodeString which is encoded with UTF-16.