Bilingual Linker – add link between multisites

I would like to set up 2 blingual multisites using Cyberchimps ifeature pro theme.

I have successfully set up sites and activared Bilingual Linker plugin.

My problem is which file to add the Bilingual Linker php code to, to add link between sites? Options seem to be header or functions but there are several on the network, site etc

Can you advise

Many thanks