I program mainly java for a living. However I really think that python gives you the biggest bang for buck.

Chronologically, my programming language exposure progression

Pascal -> Fortran -> C -> C++ -> python -> java.

My OS exposure progression CP/M -> VMS -> Unix -> MSDOS -> SCO Xenix -> Windows -> Linux -> OS/2

Other keywords.

java OAuth/OAuth 2.0

python/jython/django webservices/REST API/SOAP

C/C++/javascript/jsp/sh/SQL systems/solutions/design/ETL

cucumber/selenium/jUnit/sonarqube agile/XP (Extreme Programming)/BDD/TDD/FDD/scrum/UML

databases (oracle/postgres/sqlite/MySQL/SQL Server/Access) developer/analyst/team leader/tester/architect/designer/devops

AWS (Connect/Lambda/Lex/Pinpoint/Dynamo/SQS/API Gateway/Cloudwatch/Textract) jinja2/requests/boto3/suds/flask/pyinstaller/sockets/ladon/beautiful soup

git/subversion/clearcase/maven/ant/zappa/serverless/cvs/rcs/make jenkins/confluence/jira/bugzilla/jspwiki/github

J2EE/weblogic/jboss/glassfish/tomcat/apache Genesys PSDK/Cisco IOS API

SSML/JSON/XML/YAML/HTML Linux/Unix/MS-Windows

apache camel/struts shell scripting

docker