Now that the quality of Duolingo has fallen (even more) due to AI and people are more willing to make the jump here are just some alternative apps and what languages they have:

"I just want an identical experience to DL"

Busuu (Languages: Spanish, Japanese, French, English, German, Dutch, Italian, Portuguese, Chinese, Polish, Turkish, Russian, Arabic, Korean)

"I want a good audio-based app"

Language Transfer (Languages: French, Swahili, Italian, Greek, German, Turkish, Arabic, Spanish, English for Spanish Speakers)

"I want a good audio-based app and money's no object"

Pimsleur (Literally so many languages)

Glossika (Also a lot of languages, but minority languages are free)

*anecdote: I borrowed my brother's Japanese Pimsleur CD as a kid and I still remember how to say the weather is nice over a decade later. You can find the CDs at libraries and "other" places I'm sure.

"I have a pretty neat library card"

Mango (Languages: So many and the endangered/Indigenous courses are free even if you don't have a library that has a partnership with Mango)

Transparent Language: (Languages: THE MOST! Also the one that has the widest variety of African languages! Perhaps the most diverse in ESL and learning a foreign language not in English)

"I want SRS flashcards and have an android"

AnkiDroid: (Theoretically all languages, pre-made decks can be found easily)

"I want SRS flashcards and I have an iphone"

AnkiApp: It's almost as good as AnkiDroid and free compared to the official Anki app for iphone

"I don't mind ads and just want to learn Korean"

lingory

"I want an app made for Mandarin that's BETTER than DL and has multiple languages to learn Mandarin in"

ChineseSkill (You can use their older version of the course for free)

"I don't like any of these apps you mentioned already, give me one more"

*sob*

Theres one for sheep too if you need that