The parameter list in `@params` can have names matching the names of the parameters used by the `sp_execute_external_script` procedure itself, like `@script` or `@language`, for instance. There would need to be a way, then, to determine whether a parameter pertains to the system SP or to the user script.

Making it a rule that `@params` be always specified last in the list of the parameters pertaining to `sp_execute_external_script` and all the parameters specified after `@params` be considered pertaining to the user script is one easy way to implement the distinction. So, if a parameter like `@language` is specified after `@params`, it will be interpreted as a script parameter and not the SP's parameter.

And I believe that is what is happening in the cases where you are specifying `@output_data_1_name` or `@language` after `@params`. Those parameters simply must go before `@params` to avoid potential ambiguity.

Granted, there is not a word about this [in the manual][1], which, however, is a problem with the manual more than a problem with `sp_execute_external_script`.


  [1]: https://msdn.microsoft.com/en-US/library/mt604368.aspx "sp_execute_external_script (Transact-SQL)"