How to access custom fields master table in developer console

If you need to be able to read and write the proper values, you need a describe object call (describeSObject). This is available in SOAP, REST, and the UI-API. For example, you could call /services/v48.0/sobjects/Opportunity/describe to get the describe information.

In any case, you'll get the field and all possible API values, and their respective labels to present to a user. If you just need a translated value directly from a query (read-only access), you can use SOQL's toLabel method:

SELECT toLabel(Tags__c) FROM Opportunity