Some have pointed out that French is almost always longer. It's a bit more complicated than that. French uses a wider vocabulary than English, and uses many different words to convey different connotations. Words, as a result, tend to be longer, because they carry more information.
English tends to be much more modular and flexible. Nouns can be made into adjectives, adverbs and verbs rather easily, and "prepositions" drastically alter the meaning of verbs.
The end result is that English can be much shorter than French when trying to be concise. A short UI message will always be much shorter in English than in French. However, when conveying nuanced ideas, I believe they will be much closer in length, with perhaps a small advantage for French.