Every language, and what it asks of a speaker
Seventeen languages, each with an authored CEFR syllabus, five tutors with real regional accents, and a written set of the pronunciation patterns English speakers actually get wrong in that specific language. Regional varieties are separate languages rather than settings.
Arabic (MSA)Arabic
Arabic asks you to make sounds English has no name for, in places in your throat you have never deliberately used. That is an ear-and-mouth problem, and reading about it does very little.
Dutch
Dutch looks like it should be easy for an English speaker, and the grammar mostly is. The sounds are not, and everybody in the Netherlands will switch to English the moment you hesitate.
French
French written down and French spoken at speed are nearly different languages. Half the letters are silent, the other half link across word boundaries, and nothing on the page tells you which.
German
German rewards the person who says the sentence badly and finishes it. The grammar is front-loaded and the verb is at the end, so you have to commit before you know how it lands.
HindiDevanagari
Hindi makes four distinctions where English makes one, and all four are in the first word you will try to say. This is an ear problem before it is anything else.
Indonesian
Indonesian is the most learnable language in this app by a comfortable margin. No tones, no cases, no genders, no verb conjugation, and a spelling system that tells the truth.
Italian
Italian is friendlier to a beginner than almost any language in Europe, right up until you meet the double consonants, which are not decoration and do change the word.
JapaneseHiragana, katakana and kanji
Japanese punishes the things a text-based app cannot see. Vowel length changes the word, the rhythm is counted in beats rather than stresses, and politeness is carried in the ending you were probably going to leave off.
KoreanHangul
Hangul takes an afternoon and then lies to you for a year. What is written and what is said diverge through a stack of sound-change rules nobody puts in the first textbook.
Portuguese (Brazil)
Brazilian Portuguese is one of the most rewarding languages to speak badly. People meet you more than halfway, which is lucky, because the nasal vowels take a while.
Portuguese (Portugal)
European Portuguese is the language most badly served by apps. Nearly all of them teach Brazilian and offer Portugal as an accent, which is roughly like learning Glaswegian from a Texan.
RussianCyrillic
Russian stress is unwritten, unpredictable, and changes which vowels you say. Get it wrong and the whole word distorts, which is why reading it aloud is harder than reading it.
Spanish (Spain)
Most apps teach one Spanish and hope. If you are learning for Spain, the c in gracias is a th, the plural you is vosotros, and a car is a coche. Those are not details you correct later.
Spanish (Latin America)
Latin American Spanish is not one accent, and pretending it is does you no favours. Ustedes for every plural you, seseo everywhere, and a tutor from the region you actually care about.
ThaiThai
Thai is tonal, and a tone is not an accent detail. Say the same syllable with a different pitch contour and it is a different word, which is why a text transcript is nearly useless for correcting you.
Turkish
Turkish is beautifully regular, which is a genuine relief, right up until you meet a word with five suffixes on it and have to produce the whole thing in one breath.
Vietnamese
Vietnamese uses the Latin alphabet, which makes it look approachable. Then you meet six tones, two of which differ by a glottal break in the middle of the vowel.