Can someone improve this C# code
Your code should work ok. However, I would have used a list to store the intermediate results, and then string.Join to create the result. (Also perhaps Linq to filter the data, but then someone not familiar with Linq will have trouble maintaining the code.) Something like

string[] columnN = AnotherString.Split(',');
string[] stringList = new string[columnN.Length];

for (int i = 0; i &lt; columnN.Length; i++)
{
    string cN = columnN[i];
    string resource = Res.ResourceManager.GetString(cN.ToLower().Trim()));
    stringList[i] = resource ?? cN;
}

AnotherString = string.Join(",", stringList);