Language Difficulty Ranking for English Speakers (Foreign Service Institute)
Category 0: 

EnglishCategory I: 23-24 weeks (575-600 hours)
Languages closely related to English

Afrikaans
Danish
Dutch
French
Italian
Norwegian
Portuguese
Romanian
Spanish
SwedishCategory II: 30 weeks (750 hours)
Languages similar to English

GermanCategory III: 36 weeks (900 hours)
Languages with linguistic and/or cultural differences from English

Indonesian
Malaysian
SwahiliCategory IV: 44 weeks (1100 hours)
Languages with significant linguistic and/or cultural differences from English

Albanian
Amharic
Armenian
Azerbaijani
Bengali
Bosnian
Bulgarian
Burmese
Croatian
Czech
Estonian (*)
Finnish (*)
Georgian (*)
Greek
Hebrew
Hindi
Hungarian (*)
Icelandic
Khmer
Lao
Latvian
Lithuanian
Macedonian
Mongolian (*)
Nepali
Pashto
Persian (Dari, Farsi, Tajik)
Polish
Russian
Serbian
Sinhala
Slovak
Slovenian
Tagalog
Thai (*)
Turkish
Ukrainian
Urdu
Uzbek
Vietnamese (*)
Xhosa
ZuluCategory V: 88 weeks (2200 hours)
Languages which are exceptionally difficult for native English speakers

Arabic
Cantonese (Chinese)
Mandarin (Chinese)
Japanese (*)
Korean(*) More difficult for native English speakers to learn than other languages in the same category.

Source: https://www.state.gov/bureaus-offices/under-secretary-for-management/foreign-service-institute/
Own work
Nagihuin