How to mount a 1/2" UNF drill chuck on an angle grinder with M14 thread?
Make sure that your drill chuck is indeed a 1/2"-20 UNF thread. Because threaded drill chucks come in several variants (source):


3/8"-24 UNF thread
1/2"-20 UNF thread
5/8"-16 UNF thread (or maybe that should be 5/8"-19 UNF?)
M12x1.25


If it's indeed a 1/2"-20 UNF thread, here are several options, using varying amounts of commercial parts and DIY:


Buy the required adapter. They are rare though, but you can try your luck with one of these:


Milwaukee 4932430464 (also here) ("Winkelschleifer-Adapter M14 für Lochsägen-Gewinde 1/2"x20", GTIN/EAN 4002395381258)
Festool 484291 ("M14 Stirring Rod Adaptor 1/2", GTIN/EAN 4014549088500) Discontinued product, but still rarely found, like here.

Combine a male/male adapter and a nut. So, you buy a "M14 male to 1/2"x20 UNF male" and a long (40-50 mm) M14 nut. For the thread adapter, you can try get hold of any of the following:


Ruko 108115 ("Adapter mit Gewindeschaft M 14 für Bohrfutter mit Artikel-Nr. 108 116", GTIN/EAN 4007140077967)
Festool 769150 ("Adapter MA M14-1/2"x20")
Collomix Adapter 1/2" outside – M14 outside
Eibenstock Adapter 1/2" to M14
Make your own, by welding M14 and 1/2"-20 UNF bolts back-to-back, with their heads together.

Weld a 1/2" UNF bolt into a long M14 nut, and cut off the head of the bolt. But that could be difficult to get perfectly straight.
Cut off the M14 bolt from the angle grinder, and weld on a 1/2" UNF bolt instead. This could also be difficult to get perfectly straight when rotating, and "damaging" the angle grinder will not be acceptable in most cases. Except where you have one to spare, or it's just the angle gear from a broken angle grinder that you are building an adapter for your power drill from.
Or use your lathe. If you have one around :) to make the adapter.