Plateforme (plate-forme) is semi-accepted French, it’s an anglicisme.
Wiktionary's "etymology" section is great for 98% of words: https://en.wiktionary.org/wiki/platform#Etymology
In general, Modern English has many words from French or other Romance (Latin-origin) languages due to French being an important vector through which Latin-origin words entered English, evolving it in competition with Germanic-origin words.