Okay, so I get it now. I think.
It works fine if you define your main (default) language translation in .php files. Then only use .json files for the other languages, the fallback to .php works. You just have to remember to define the json differently to the php, for example:
validation.php
'multiple_of' => 'The :attribute must be a multiple of :value.',
ru.json
"validation.multiple_of" : "Russian transaltion here",