I have a custom field of type long text on an image file type field used in a custom content type. The content type, image file field and my custom field on the file have translation turned on. When adding new/editing the file only the default language is possible to edit.
If I add a node using said content type, add a new image and type in text in my custom field, after saving and translating the node, when editing the file to add translated content to the field, the original language is edited. The text area name for the field has the default language code:
<textarea class="text-full form-textarea" id="edit-field-arkivbilde-tekst-nb-0-value" name="field_arkivbilde_tekst[nb][value]" cols="60" rows="5" aria-hidden="true" style="visibility: hidden; display: none;">..content..</textarea>