It's most closely related to Estonian.
Estonian, and Saami (Lapp)
estonian
Maybe Estonian
The language most closely related to English is Frisian, a language spoken in the Netherlands and Germany.
Estonian language have lots of similar words with a bit different meanings. estonian nisu = wheat finnish nisu = bun estonian piima = milk finnish piimä = sour milk
English is most closely related to the Germanic language family, specifically the West Germanic branch.
The language most closely related to Vietnamese is Muong, which is a language spoken by the Muong ethnic group in Vietnam. Both Vietnamese and Muong belong to the Vietic branch of the Austroasiatic language family.
I think the most expressive language is Hebrew or Finnish.
Iran
indo-european
The most frequently used letter in the Finnish language would be the letter "A".
The closest widely known language to Hungarian is Finnish, but still they are very far from each other nowadays. There more closely related languages to Hungarian (from the Ugric branch of Finno-Ugric languages, Finnish is in the Finno-[Permic] branch, of course), but those languages are highly unknown for most people also those languages are spoken by only a smaller group of people (at least compared to Hungarian and Finnish which means millions). Anyway, more closely related languages of Hungarian includes (for example): Mansi and Khanty.
Neither, it's a Ural-Altaic language more related to central and northern asian languages than most european ones, which are Indo-European
Japanese doesn't have an actual language group like most languages do (e.g. Danish is part of the North Germanic language group, closely related to Swedish and Norwegian), however, I have heard that Korean would be the most closely related, as both languages borrowed common characters from the chinese language and also my guess would be they are reasonably similar given they are geographically close, however, by no means mutually intelligible.