Sharing Config between Node.js, React.js and Python Apps
In my mind there are two main approaches you'll see in the wild. 

Config file. As nc. mentioned, you could make a TOML file that all of the different tools and applications can access. This is nice since there is only one source of truth and TOML is very human readable. The cons are having to find a TOML parser for your language (imo not that difficult), and making sure the file is there when a script/service/whatever expects it. I've gone this route and its not that bad, you can even put the file in "the cloud" (NAS, shared data store, folder with broad read permissions), and have it easily accessible if you need to spin up more than one instance of a service. 
Environment variables. I've liked this approach lately as it is always the minimal amount of config you could possibly do and pretty much any language has facilities for accessing environment variables. Also, you don't have possibly sensitive information hanging around in a publicly accessible plain text file. 

For python you would call os.environ[MY_CONFIG_NAME] and presto, you have the config info.
Both approaches are common and have plenty of support in almost every language.
As far as client, server separation. I tend to make my clients as "dumb" as possible and my servers as "smart" as possible.
My project currently involves several components using either Javascript and Python 

Node.js server
Python flask API server
Python data processing scripts (numpy, pandas, etc)
React.js web client
React Native mobile client

These components share several common config values such as a database connection URI string postgresql://admin:password@localhost:5432/foobar used by server-side components and the API server url https://api.example.com/v1 used by client-side components.
Question: Is there a best practice for sharing config files across components that uses different languages? 
For example, 

Should all components (server and client side) access the same config file? If so, which format should it be in, JSON, YAML, etc? 
Should all components share a common config file (where only common config values are stored in), but also have a second config file specific to that component?
Should all server side components share the same config file, while client side components have their own individual config files?

Thank you for your advices :)