Nationalization and nationalisation are both English terms.
Nationalization is predominantly used in 🇺🇸 American (US) English (en-US) while nationalisation is predominantly used in 🇬🇧  British English (used in UK/AU/NZ) (en-GB).
In terms of actual appearance and usage, here's a breakdown by country, with usage level out of 100 (if available) 👇:
| Term | US | UK | India | Philippines | Canada | Australia | Liberia | Ireland | New Zealand | Jamaica | Trinidad & Tobago | Guyana | 
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| nationalization | 94 | 10 | 32 | 96 | 47 | 20 | 0 | 18 | 0 | 100 | 0 | 0 | 
| nationalisation | 6 | 90 | 68 | 4 | 53 | 80 | 0 | 82 | 100 | 0 | 0 | 0 | 
Below, we provide some examples of when to use nationalization or nationalisation with sample sentences.
(Examples are Wikipedia snippets under the CC ShareAlike 3.0 license.)