How do add a condition to check if created node is language source (in a multilingual site)? I need to fire a rule action when a node is created and this node is the original source in a multilingual site. This means that this node is not the translation of other node.

How do add a condition to check if the created node the original source in a multilingual site?
Check your created node if it has a $node->tnid. The tnid (translation id) would be not null if its a translation from another node.