In addition to what other people posted, I use the Teach Yourself language learning series, which includes reading, listening, and writing exercises, in addition to any online resources I can find that include interactive language learning mediums. Some universities have free or inexpensive options to learn various languages. Google Translate is also helpful for learning pronunciation, dictation, and spelling
Here is the link for Teach Yourself. You can get their books on Amazon, and use their app/website for listening exercises:
https://library.teachyourself.com/
FWIW, I grew up speaking Portuguese; and have formally studied Spanish and Arabic. Now I am learning Icelandic, Finnish, and Persian.