Can we test this with pure core?
I am not aware of a frontend modal showing sorting fields for articles (Select Article only displays Search Tools afaik);

If this is only for your extension, don't you think we may have unwanted overrides between the frontend and backend lang strings if they have the same constants but variations in the values + loading the entire ini for just a few strings?
Would not it be simpler to just add the missing sorting strings in frontend or, as it concerns only your extension, in the extension ini file?

Here is only com_content involved as it is the modal form field which get's loaded. I don't think there side effects for it in this particular modal view. But to be 100% save the strings should be moved added to the front end ini file.

But to be 100% save the strings should be moved to the front end ini file.

I guess you meant added ?
I would prefer that solution imho if it is really useful for 3pd.

Can you point me into the direction how I can identify them as fast as possible, then I will change the pr accordingly.

Very few are in com_content, most are in main ini file:

<option value="">JGLOBAL_SORT_BY</option>
			<option value="a.ordering ASC">JGRID_HEADING_ORDERING_ASC</option>
			<option value="a.ordering DESC">JGRID_HEADING_ORDERING_DESC</option>
			<option value="a.state ASC">JSTATUS_ASC</option>
			<option value="a.state DESC">JSTATUS_DESC</option>
			<option value="a.featured ASC">JFEATURED_ASC</option>
			<option value="a.featured DESC">JFEATURED_DESC</option>
			<option value="a.title ASC">JGLOBAL_TITLE_ASC</option>
			<option value="a.title DESC">JGLOBAL_TITLE_DESC</option>
			<option value="category_title ASC">JCATEGORY_ASC</option>
			<option value="category_title DESC">JCATEGORY_DESC</option>
			<option value="a.access ASC">JGRID_HEADING_ACCESS_ASC</option>
			<option value="a.access DESC">JGRID_HEADING_ACCESS_DESC</option>
			<option value="association ASC" requires="associations">JASSOCIATIONS_ASC</option>
			<option value="association DESC" requires="associations">JASSOCIATIONS_DESC</option>
			<option value="a.created_by ASC">JAUTHOR_ASC</option>
			<option value="a.created_by DESC">JAUTHOR_DESC</option>
			<option value="language ASC">JGRID_HEADING_LANGUAGE_ASC</option>
			<option value="language DESC">JGRID_HEADING_LANGUAGE_DESC</option>
			<option value="a.created ASC">JDATE_ASC</option>
			<option value="a.created DESC">JDATE_DESC</option>
			<option value="a.modified ASC">COM_CONTENT_MODIFIED_ASC</option>
			<option value="a.modified DESC">COM_CONTENT_MODIFIED_DESC</option>
			<option value="a.publish_up ASC">COM_CONTENT_PUBLISH_UP_ASC</option>
			<option value="a.publish_up DESC">COM_CONTENT_PUBLISH_UP_DESC</option>
			<option value="a.publish_down ASC">COM_CONTENT_PUBLISH_DOWN_ASC</option>
			<option value="a.publish_down DESC">COM_CONTENT_PUBLISH_DOWN_DESC</option>
			<option value="a.hits ASC">JGLOBAL_HITS_ASC</option>
			<option value="a.hits DESC">JGLOBAL_HITS_DESC</option>
			<option value="rating_count ASC" requires="vote">JGLOBAL_VOTES_ASC</option>
			<option value="rating_count DESC" requires="vote">JGLOBAL_VOTES_DESC</option>
			<option value="rating ASC" requires="vote">JGLOBAL_RATINGS_ASC</option>
			<option value="rating DESC" requires="vote">JGLOBAL_RATINGS_DESC</option>
			<option value="a.id ASC">JGRID_HEADING_ID_ASC</option>
			<option value="a.id DESC">JGRID_HEADING_ID_DESC</option>

That was for backend filter.
In frontend aricles filters are

<option value="">JGLOBAL_SORT_BY</option>
			<option value="a.ordering ASC">JGRID_HEADING_ORDERING_ASC</option>
			<option value="a.ordering DESC">JGRID_HEADING_ORDERING_DESC</option>
			<option value="a.state ASC">JSTATUS_ASC</option>
			<option value="a.state DESC">JSTATUS_DESC</option>
			<option value="a.title ASC">JGLOBAL_TITLE_ASC</option>
			<option value="a.title DESC">JGLOBAL_TITLE_DESC</option>
			<option value="category_title ASC">JCATEGORY_ASC</option>
			<option value="category_title DESC">JCATEGORY_DESC</option>
			<option value="association ASC" requires="associations">JASSOCIATIONS_ASC</option>
			<option value="association DESC" requires="associations">JASSOCIATIONS_DESC</option>
			<option value="a.access ASC">JGRID_HEADING_ACCESS_ASC</option>
			<option value="a.access DESC">JGRID_HEADING_ACCESS_DESC</option>
			<option value="a.created_by ASC">JAUTHOR_ASC</option>
			<option value="a.created_by DESC">JAUTHOR_DESC</option>
			<option value="language ASC">JGRID_HEADING_LANGUAGE_ASC</option>
			<option value="language DESC">JGRID_HEADING_LANGUAGE_DESC</option>
			<option value="a.created ASC">JDATE_ASC</option>
			<option value="a.created DESC">JDATE_DESC</option>
			<option value="a.id ASC">JGRID_HEADING_ID_ASC</option>
			<option value="a.id DESC">JGRID_HEADING_ID_DESC</option>
			<option value="a.featured ASC">JFEATURED_ASC</option>
			<option value="a.featured DESC">JFEATURED_DESC</option>
			<option value="a.hits ASC">JGLOBAL_HITS_ASC</option>
			<option value="a.hits DESC">JGLOBAL_HITS_DESC</option>

As I have no idea which one you use in your extension, can't help further.

@laoneo should the Status be "Needs Review"?

As is, it can't be merged as I pointed above:

don't you think we may have unwanted overrides between the frontend and backend lang strings if they have the same constants but variations in the values + loading the entire ini for just a few strings?

I'm closing it because of lack of time at the moment.