// getting macro placeholders from PCTLoop
var macroPlaceHolders = pctLoop.MacroPlaceHolders;

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

// changing every variable value
foreach (var macroPlaceHolder in macroPlaceHolders)
{
    foreach (var variable in macroPlaceHolder.VariableNames)
    {
        macroPlaceHolder[variable] = mls;
    }
}

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