Magento – Magento 2 translate custom options

Product data is not translated with CSV files per language, but stored per store view. Each store view has one language, so you should use the store specific values for translation.

Now this not only applies to product attributes but also to labels of custom options.

3. In the upper-left corner set Store View to the view for the translation. When prompted to confirm, tap OK.

4. For each field to be edited, do the following:

   a. Clear the Use Default Value checkbox to the right of the field.

   b. Either paste or type the translated text into the field.

   Make sure to translate all text fields, including image labels and Alt text, Search Engine Optimization fields and any Custom Options information.

5. When complete, tap Save.

^{Source: Magento 2.0 User Guide}