How to filter the output of custom loops and plugins

Could you please point to any resources on how to:

1) Filter the output of a custom loop to only show posts for the current lang?
2) The same for a widget – Im in conversation with a plugin dev, if possible would like to provide him some info on what the plugin should be looking at to also only show contents from the current lang.

Thanks in advance for any help.

Permalinks – https://multisite.dev/wip053/sample-post/
Static Front Page – Yes
Polylang Settings:
– The language is set from content
– Remove /language/ in pretty permalinks
– The front page url contains the language code instead of the page name or page id
– Activate languages and translations for media
– Sync for Taxonomies, Page template, Post format, Featured image

https://www.ads-software.com/plugins/polylang/