how to insert multiple values into a table cell in sql server

You likely don't want to do this, as you'll be breaking even first normal form. Instead, it may make more sense to model the relationship as a many-to-many.

In this case you would have a Languages table, an (e.g.) User table, and then a junction table (e.g. LanguagesKnown) which links many languages to many users using two foreign keys for each row in the junction table.

You could use a simple loop:

string sql = "INSERT INTO dbo.Language VALUES(@languageID, @languageName);";
using (var con = new SqlConnection(yourConnectionString))
using(var cmd = new SqlCommand(sql, con))
{
    con.Open();
    foreach (ListItem item in lbLanguagesKnown.Items)
    {
        cmd.Parameters.Clear();
        if (item.Selected)
        {
            cmd.Parameters.AddWithValue("@languageID", int.Parse(item.Value));
            cmd.Parameters.AddWithValue("@languageName", item.Text);
            int insertedCount = cmd.ExecuteNonQuery();
        }
    }
}

I have used sql-parameters to prevent sql-injection. The using-statement is used to ensure that the connection gets closed as soon as possible (even in case of an exception).

However, the requirement makes little sense. Why do you want to insert masterdata when the user selects part of it. Sounds as if you actually want to insert in a different table which links to the language table.

For example:

Table1: UserLanguages with UserID + LanguageID

Table2: Languages with above model