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

This is how i write something on text file:

Code:

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

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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... ?

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

while 4 is the line :)