You are excited about Joomla custom fields and you want to add them to your multi-lingual site - then you suddenly realise that you don't know how to make the custom fields multi-lingual.
You know you could add different versions of the same field - specifying the language for each of them. The downside of this is that it becomes complex when you want to control the layout of your articles - with so many field and language combinations the template overrides quickly become difficult to manage.
If you use separate custom fields for each language then the relevant ones would be included in your edit pages but filtering and ordering your articles based on these custom fields then becomes very cumbersome to manage.
There is another approach - using the Joomla language overrides system.