How can I detect the OS language?

You can get all kinds of stuff from the regional settings. Here is an example. Play with it....
VB Code:
Private Declare Function GetLocaleInfo Lib "kernel32" Alias "GetLocaleInfoA" (ByVal Locale As Long, ByVal LCType As Long, ByVal lpLCData As String, ByVal cchData As Long) As Long
Private Declare Function SetLocaleInfo Lib "kernel32" Alias "SetLocaleInfoA" (ByVal Locale As Long, ByVal LCType As Long, ByVal lpLCData As String) As Boolean
Private Declare Function GetUserDefaultLCID% Lib "kernel32" ()
 
Private Const LOCALE_ICENTURY = &H24
Private Const LOCALE_ICOUNTRY = &H5
Private Const LOCALE_ICURRDIGITS = &H19
Private Const LOCALE_ICURRENCY = &H1B
Private Const LOCALE_IDATE = &H21
Private Const LOCALE_IDAYLZERO = &H26
Private Const LOCALE_IDEFAULTCODEPAGE = &HB
Private Const LOCALE_IDEFAULTCOUNTRY = &HA
Private Const LOCALE_IDEFAULTLANGUAGE = &H9
Private Const LOCALE_IDIGITS = &H11
Private Const LOCALE_IINTLCURRDIGITS = &H1A
Private Const LOCALE_ILANGUAGE = &H1
Private Const LOCALE_ILDATE = &H22
Private Const LOCALE_ILZERO = &H12
Private Const LOCALE_IMEASURE = &HD
Private Const LOCALE_IMONLZERO = &H27
Private Const LOCALE_INEGCURR = &H1C
Private Const LOCALE_INEGSEPBYSPACE = &H57
Private Const LOCALE_INEGSIGNPOSN = &H53
Private Const LOCALE_INEGSYMPRECEDES = &H56
Private Const LOCALE_IPOSSEPBYSPACE = &H55
Private Const LOCALE_IPOSSIGNPOSN = &H52
Private Const LOCALE_IPOSSYMPRECEDES = &H54
Private Const LOCALE_ITIME = &H23
Private Const LOCALE_ITLZERO = &H25
Private Const LOCALE_NOUSEROVERRIDE = &H80000000
Private Const LOCALE_S1159 = &H28
Private Const LOCALE_S2359 = &H29
Private Const LOCALE_SABBREVCTRYNAME = &H7
Private Const LOCALE_SABBREVDAYNAME1 = &H31
Private Const LOCALE_SABBREVDAYNAME2 = &H32
Private Const LOCALE_SABBREVDAYNAME3 = &H33
Private Const LOCALE_SABBREVDAYNAME4 = &H34
Private Const LOCALE_SABBREVDAYNAME5 = &H35
Private Const LOCALE_SABBREVDAYNAME6 = &H36
Private Const LOCALE_SABBREVDAYNAME7 = &H37
Private Const LOCALE_SABBREVLANGNAME = &H3
Private Const LOCALE_SABBREVMONTHNAME1 = &H44
Private Const LOCALE_SCOUNTRY = &H6
Private Const LOCALE_SCURRENCY = &H14
Private Const LOCALE_SDATE = &H1D
Private Const LOCALE_SDAYNAME1 = &H2A
Private Const LOCALE_SDAYNAME2 = &H2B
Private Const LOCALE_SDAYNAME3 = &H2C
Private Const LOCALE_SDAYNAME4 = &H2D
Private Const LOCALE_SDAYNAME5 = &H2E
Private Const LOCALE_SDAYNAME6 = &H2F
Private Const LOCALE_SDAYNAME7 = &H30
Private Const LOCALE_SDECIMAL = &HE
Private Const LOCALE_SENGCOUNTRY = &H1002
Private Const LOCALE_SENGLANGUAGE = &H1001
Private Const LOCALE_SGROUPING = &H10
Private Const LOCALE_SINTLSYMBOL = &H15
Private Const LOCALE_SLANGUAGE = &H2
Private Const LOCALE_SLIST = &HC
Private Const LOCALE_SLONGDATE = &H20
Private Const LOCALE_SMONDECIMALSEP = &H16
Private Const LOCALE_SMONGROUPING = &H18
Private Const LOCALE_SMONTHNAME1 = &H38
Private Const LOCALE_SMONTHNAME10 = &H41
Private Const LOCALE_SMONTHNAME11 = &H42
Private Const LOCALE_SMONTHNAME12 = &H43
Private Const LOCALE_SMONTHNAME2 = &H39
Private Const LOCALE_SMONTHNAME3 = &H3A
Private Const LOCALE_SMONTHNAME4 = &H3B
Private Const LOCALE_SMONTHNAME5 = &H3C
Private Const LOCALE_SMONTHNAME6 = &H3D
Private Const LOCALE_SMONTHNAME7 = &H3E
Private Const LOCALE_SMONTHNAME8 = &H3F
Private Const LOCALE_SMONTHNAME9 = &H40
Private Const LOCALE_SMONTHOUSANDSEP = &H17
Private Const LOCALE_SNATIVECTRYNAME = &H8
Private Const LOCALE_SNATIVEDIGITS = &H13
Private Const LOCALE_SNATIVELANGNAME = &H4
Private Const LOCALE_SNEGATIVESIGN = &H51
Private Const LOCALE_SPOSITIVESIGN = &H50
Private Const LOCALE_SSHORTDATE = &H1F
Private Const LOCALE_STHOUSAND = &HF
Private Const LOCALE_STIME = &H1E
Private Const LOCALE_STIMEFORMAT = &H1003
 
Private Sub GetRegionalSettings() ' Retrieve the regional setting
 
      Dim Symbol As String
      Dim iRet1 As Long
      Dim iRet2 As Long
      Dim lpLCDataVar As String
      Dim Pos As Integer
      Dim Locale As Long
      
      Locale = GetUserDefaultLCID()
 
'LOCALE_SDATE is the constant for the date separator
'as stated in declarations
'for any other locale setting just change the constant
 
'Function can also be re-written to take the
'locale symbol being requested as a parameter
      
      iRet1 = GetLocaleInfo(Locale, LOCALE_SDATE, _
      lpLCDataVar, 0)
      Symbol = String$(iRet1, 0)
      
      iRet2 = GetLocaleInfo(Locale, LOCALE_SDATE, Symbol, iRet1)
      Pos = InStr(Symbol, Chr$(0))
      If Pos > 0 Then
           Symbol = Left$(Symbol, Pos - 1)
           MsgBox "Regional Setting = " + Symbol
      End If
 
End Sub
 
Private Sub SetRegionalSettings() 'Change the regional setting
 
      Dim Symbol As String
      Dim iRet As Long
      Dim Locale As Long
      
'LOCALE_SDATE is the constant for the date separator
'as stated in declarations
'for any other locale setting just change the constant
 
'Function can also be re-written to take the
'locale information being set as a parameter
 
      Locale = GetUserDefaultLCID() 'Get user Locale ID
      Symbol = "-" 'New character for the locale
      iRet = SetLocaleInfo(Locale, LOCALE_SDATE, Symbol)
     
End Sub