(1) End user languages used by users that would like to access the app in that language, but do not need access to Setup. Example: a company has its headquarters in an English-speaking country, but has non-admin employees in other countries who would prefer to use the app in a language supported as an EUL. Since they don't need access to Setup, the EUL provides enough translations out of the box to fit their needs.
Platform languages allow organizations to provide their own translations, via the Translation Workbench, for languages that Salesforce doesn't provide any translations for. The example would be the same as above, except that the company would need to provide their own translations for their end users.
(2) All languages are available in Translation Workbench. To enable additional languages, go to Setup > Company Information > Language Settings. Click the checkboxes to enable End User and Platform languages. Then, in Setup > Translation Workbench > Translation Settings, you can add available languages to the workbench.