I wrote a couple iOS apps to help learn 100 words in several languages.

Hundred Words: https://apps.apple.com/us/app/hundred-words/id1469449237

Language Pairs: https://itunes.apple.com/us/app/language-pairs/id1438817614?...

DuoLingo is probably the most widely used app.

https://www.duolingo.com/

Get somewhat proficient in basic vocabulary then travel. Immersion is the best way to learn but it helps to know some basic vocabulary.

youtube. Find out how to translate a term you are interested in to the target language and search that term. You will find a ton of videos of someone in your target language doing something of interest to you. For example "Weld" in Spanish is "Soldar" - throw that into the search box and I get plenty of interesting videos of people teaching how to weld, which is a topic I'm interested in so I enjoy watching them and I have some idea of what they must be talking about. (Of course some of them are giving bad advice, but I'm there to learn the language...) Don't forget to look at the related videos from the videos you watch to branch out topics.

You need to find your own topic of interest, but it is a great way to learn from native speakers as they would speak.

Assimil is the best program I've found so far for learning a foreign language. The way you learn the language feels very natural to me. I used their book but they've recently released an app which may be good.

I also recommend Pimsleur as a supplement to work on your accent and listening skills. Don't let the limited vocabulary misguide you...Pimsleur teaches and emphasizes a lot of very useful language patterns/structures. Great tool in the language learning toolbox

Read the book Fluent Forever, and download Anki, I learned Italian and French using this method. Duolingo is pretty useless.

In my view, apps are ok to train vocabulary, but not really to learn the grammar and usage patterns. As such, some languages might lend themselves somewhat better to learn via apps (Malay), while others are less suitable (Russian, Japanese).

Find native speakers and talk to them. Write down words you don't rememberand put them in Anki (or Pleco flash cards if you're learning Mandarin) and drill yourself on them daily.