You would just say the Kostakis family.
The only correct plural for French is French. "Frenches", as you say, or 'Frenchies' is a derogatory slang term.
Francises Francis's would be if they own something e.g. The Francis's TV
It would be... example: Johnsons the correct spelling would be Johnsons'
The plural form of the proper noun Diaz is the Diazes.The plural possessive form is Diazes'.Example: The Diazes' children are twins.
The Erlich family of Southern California spells the plural Erlichs. See the link below.
'les Duclos' Tonight we dine at the Duclos' > ce soir nous mangeons chez les Duclos. There is no plural spelling for last names in French : write 'les Bonaparte, les Dupont, les Dumoulin'
The Bushes
Without the last E (i.e., tablecloths). Cloths for the table, not clothes for the table.
The job belongs to last year as it was, so it is a possessive. So there is an apostrophe needed between year and s, so it does not indicate a plural. The correct form is: Last year's job.
The plural is Averys.
The plural of the last name Welsh is "Welshes."
The plural of last, which in its noun form can mean "a shoemaker's tool", as well as "the final object", would be lasts.