How to write something on a text file on a certain line?

[Bong.Da.City]

This is how i write something on text file:

{system("echo \"Hello\" > /home/$USER/.config/Wallch/Checks/language_selection");

{system("echo \"Hello\" > /home/$USER/.config/Wallch/Checks/language_selection");

To copy to clipboard, switch view to plain text mode 

But how to write something on a certain line without changing the others?
For example how can i write "Hello" on the third line of my text file without changing the first line... ?

[Bong.Da.City]

ok solved:
sed -i "4i\Hello" file_here

while 4 is the line