"This mountain is the highest in the world."
Translation:Cette montagne est la plus haute du monde.
Dans is often translated as "in" but it can have the underlying sense of "inside (of)". That doesn't seem quite right in this phrase.
I've noticed that French sometimes prefers to use a possessive, de. One way to translate this sentence is "That is the world's highest mountain." Du monde ==> "the world's" .
Omit the article for feminine countries such as: de France, d'Angleterre
Keep the article for masculine countries such as: du (=de+le) Portugal, des (=de+les) États-Unis
(source: Sitesurf "Cette montagne est la plus haute d'Europe.") Use the article for masculine countries such as: