// getting macro placeholder value sets from planning object
var oMacroPlaceHolderValueSets = oPlanningObject.MacroPlaceHolderValueSets;

// defining value as string in multiple languages
string text = "Test!!!";
MultiLangString mls = new MultiLangString();
mls.AddString(ISOCode.Language.L_en_US, text);
mls.AddString(ISOCode.Language.L_de_DE, text);

// changing every variable to defined value
foreach (var oMacroPlaceHolderValueSet in oMacroPlaceHolderValueSets)
{
    foreach (string variable in oMacroPlaceHolderValueSet.VariableNames)
    {
        oMacroPlaceHolderValueSet[variable] = mls;
    }
}

// writing data back to planning object
oMacroPlaceHolderValueSets.Store();