Populate custom field with translated content

Hi, I'm successfully getting translated posts on my blog, and now trying to populate a custom field for Yoast SEO plugin like: _yoast_wpseo_metadesc => %%item_content%% or _yoast_wpseo_metadesc => %%item_description%% It works, however I always get the content in original language. My questions are: 1. How can I submit the translated content to my custom field? 2. Is the a way to trim this content to particular length, so I can make good SEO? Thank you