Theme: Borrowed Words - Lexical borrowing is the adoption of individual words or phrases in their native language and used "as is" in our English.
39A. With 42-Across, cars like BMWs and Audis ... or 18-, 24-, 53- and 63-Across: FOREIGN. 42-Across: IMPORTS
18A. Writer's chief work (Latin): MAGNUM OPUS. (great work)
24A. Done deal (French): FAIT ACCOMPLI. (fact accomplished)
53A. English, in many non-English speaking countries (Italian): LINGUA FRANCA. Literally "Frankish tongue". Good definition by Merriam-Webster. LINK Read the "Did You Know?" section.
63A. Young sensation (German): WUNDERKIND. (a wonder child)
Argyle here. Don't shoot the messenger.
1. Ponzi scheme, e.g.: SCAM
5. Impersonated: APED
9. Your __ Highness: ROYAL. 37D. Princess' headpiece: TIARA
14. Drilled bowling ball feature: HOLE
15. Artist Magritte: RENE
16. Shah of Iran, in 1979-'80: EXILE. ??
17. Bahrain big shot: EMIR. Kingdom of Bahrain is a small Arab monarchy.
20. Sphere of influence: AMBIT. The adjective form is ambient.
22. Drinking glass edges: RIMs
23. Ambulance initials: EMS. (emergency medical service)
28. Farm country skyline highlights: SILOS
29. Nissan model: SENTRA
33. "Take me for a walk!": [ARF!]
36. Expel from office: OUST
38. Amazon's business: E-TAIL
44. Rhett's last words: A DAMN
45. Siamese, now: THAI
47. __ dye: food-coloring compound: AZO. Infrequently seen fill.
48. Live (in): RESIDE
50. Lover of Euridice, in a Gluck opera: ORFEO. Wiki Previously clued as Monteverdi title character. Not Monday friendly.
58. Word with health or day: SPA
61. Oboist's need: REED
62. Lagoon-enclosing isle: ATOLL
67. Sinister: EVIL
68. Cortés subject: AZTEC. I believe this refers to Cortés personally governing the Aztecs, making them his "subjects".
69. Wrinkle, as a brow: KNIT
70. San __, Italy: REMO
71. Suppose for argument's sake: POSIT
72. Many van Goghs: OILS
73. Killed, as a dragon: SLEW
1. Harvest bundle: SHEAF
2. Grammar class subject: COMMA. Also, in the news.
3. Legal defense mechanism?: ALIBI
4. Advantages: MERITS
5. Shortstop's asset: ARM. He has to rifle the ball to first base to beat the runner.
6. Green soup veggie: PEA
7. Thoroughly absorb: ENGROSS
8. Jeans fabric: DENIM
9. Dreaming phase: REM SLEEP. (rapid eye movement)
10. Kitchenware brand: OXO
11. Frightened exclamation: [YIPE!]
12. University fund-raising target: ALUM
13. A smaller amount of: LESS
19. Quartet assigned to bases: UMPs. In the big leagues, at least.
21. Mai __: TAI
25. Plumber's challenge: CLOG
26. Dracula's title: COUNT
27. Avid about: INTO
30. Skater Lipinski: TARA. Roller skating as a nine-year-old.
31. Nabisco cracker: RITZ
32. "Wait, there's more ... ": "ALSO ... " for a limited time.
33. Miles away: AFAR
34. Traveled by bike: RODE
35. Abbey titles: FRAs
40. Oscar winner Jannings: EMIL. The first Oscar recipient and one of the first persons you learn when doing crosswords.
41. Roundabout, as a route: INDIRECT
43. Offend slightly: MIFF
46. Entertainer who often got tied up in his work?: HOUDINI. Cute clue.
49. Part of DOE: Abbr.: ENERgy
51. Historic period: ERA
52. Many top-rated TV shows of the late '50s/early '60s: OATERS. Westerns.
54. Gordon __, "Wall Street" antagonist: GEKKO. "If you need a friend, get a dog."
55. James Joyce work: NOVEL. CSO
56. Weather, in poems: CLIME
57. Permit: ALLOW
58. Tit for tat, e.g.: SWAP
59. "The Godfather" novelist Mario: PUZO. It was on TV this weekend.
60. Teeny colonizers: ANTs
64. Agnus __: DEI. (Lamb of God)
65. Zip, in soccer: NIL
66. Wino's woe: DTs. (delirium tremens)