In light of Duolingo laying off its translators, here are my favourite language apps (primarily for Mandarin Chinese, Japanese, and te reo Māori).
Multiple Languages
Anki is a flashcard programme and app that's not exclusively for languages. While making your own decks is ideal, you can also download shared decks for most languages.
If you're learning Japanese, specifically, Seth Clydesdale has websites for practicing alongside Genki's 2nd or 3rd editions, and he also provides his own shared Anki decks for Genki.
And if you're learning te reo Māori, specifically, here's a guide on how to make your own deck.
TOFU Learn is an app for learning vocabulary that's very similar to Anki. However, it has particularly excellent shared decks for East Asian languages. I've used it extensively for practicing 汉字. Additionally, if you're learning te reo Māori, there's a shared deck of vocabulary from Māori Made Easy!
Mandarin Chinese
Hello Chinese is a fantastic app for people at the HSK 1-4 levels. While there's a paid version, the only thing paying unlocks is access to podcast lessons, which imo are not really necessary. Without paying you still have access to all the gamified lessons which are laid out much like Duolingo's lessons. However, unlike Duolingo, Hello Chinese actually teaches grammar directly, properly teaches 汉字, and includes native audio practice.
Japanese
Renshuu is a website and app for learning and practicing Japanese. The vast majority of its content is available for free. There's also a Discord community where you can practice alongside others.
Kanji Dojo is a free and open source app for learning and practicing the stroke order of kanji. You can learn progressively by JLPT level or by Japanese grades. There's also the option to learn and practice kana stroke order as well.