Available node language only English
Change interface lang to the node target lang

The configuration you describe will always only allow one language when creating new nodes: the current language of the interface. 

That is because you have Set custom language as default for new content. (and the current one as the default).

If you want to create content in other languages, you have to visit the node creation page in that target language (/fr/node/add/article if you're using prefixes). 

Or allow other langs to be chosen

But if you want to be able to choose which language to assign to a node no matter what interface language you're currently using you have to untick the   Set custom language as default for new content. while keeping Require language (Do not allow Language Neutral). 

That way your interface may be English, but you'll be able to choose any language in a drop-down to assign to that node.