On the face of it, this is a conventional cryptic bilingual crossword.  The difference this time is that it may help you to know that the unchecked letters around the perimeter can be arranged to form the improbable phrase ‘impair elastic chateau line’. Answers may be in French or English, the choice often implied in the clues.  All accents ignored.

ACROSS

1          Hold out scrap to the very end (4,5)
6          Almost replenish in return for a long-term insider (5)
9          Obliges little businesses to keep the Spanish politician in front (7)
10         Hospital department with boy about interior plumbing (7)
11         Boredom of French in unbroken nuisance (5)
12         Attachment to a very quiet final period (9)
14         Openings for ornithologists in evaluating French bird (3)
15         How to ape one bird with fifty people around (2,9).
17         Guesses about peoples taking cooperative benefit (11)
19         Head of department starts as carrier (3)
20         Relating to joints less quiet than individual (9)
22         Capek play with gangster in the country (5)
24         Hardy heroine has time to discover mosaic pieces (7)
26         Number of insects taken on by letter (7)  
27         Merry Frenchman taking bible class is short of money (5)
28         Expands in folds (9)

DOWN

1          Airman with ambassador loose in Paris (5)

2          People of Nice, perhaps, go to church  –  or seed! (7)

3          Menu expert said to colour 14 (9)
4          A matter of will to examine a kind of patient (11)
5          Tool found in Jericho excavation(3)
6          US city can supply tongue (5)
7          Iron car brings back terrorists to another car (7)
8          Creed Lane wrong to be washed again (9)
13         Tropical cut-out of the rich (11)
14         Such as Leonardo, re-encountered at school reunion? (3.6)
16         A scan sure gives confidence (9)
18         24 start makes for pace in French life (7)
19         Queen hangs out in poissonière (7)
21         Praise result of two hip operations (5)
23         Rolls for jousting (5)
25         I am one-down here, mate!  (3)