What is the best offline dictionary app for iPhone?
Check what is available offline and whether a separate download is required. App size alone does not prove dictionary coverage: compression, included data and optional packs differ. Install the resources you need, enable airplane mode, and try words from your book before relying on the app without a connection.
The second is whether the offline dictionary is the whole app or a paid extra. Several apps are free to install and gate offline use, or additional languages, behind a subscription. That is a reasonable model, but it is worth knowing before you rely on it on a plane.
The third is inflection handling, if you are using it while reading. A dictionary that cannot get from wended to wend will disappoint you on the first page of a novel.
Booktionary is built around all three: English definitions are built in, French, Spanish, Portuguese, Italian and German dictionaries are optional downloads, there is no subscription or in-app purchase, and inflected forms are reduced before searching. It is a camera dictionary rather than a typing one, which suits reading and does not suit writing.