Other than English, my language skills are less than stellar. However, I do love learning them in theory, and so my favorite language resources for each are below.
Partially for their beauty and partially for how interesting they are epigraphically, I love learning semasiographic scripts; between Egyptian, Mayan, and Chinese, I cover 3 of the known independent origins of writing. I've scratched the surface with Nahuatl, and then I'll eventually look to learn some basic cuneiform (not a language, I know, but I have it on good authority that the script is relatively easy and it was the script du jour for international relations in the Bronze Age).
Spanish
Spanishdict.com is the best website, hands down. Highly recommend. Other than that, my Spanish was largely learned without digital resources.
Egyptian Hieroglyphics
The absolute best resource I've found is this searchable dictionary (free). Note it uses European transliterations (j, not i).
I love this guy's website — it has an abbreviated version of Gardner's sign list and some great links.
If you want to actually write the hieroglyphics by hand, this guide will do wonders.
Check out my Hieroglyphics Quizlets.
Mayan Glyphs
Check out Michael Coe's books. FAMSI has great resources on its website. I made this set of Quizlets with common glyphs and words.
Chinese
I wish I had good recommendations - this section may become just recommended media. If you have better recommendations, let me know.
I love Ninchanese, which fits well with my learning style and preferences — I am prioritizing speaking, reading, and typing, because actually writing Chinese is difficult and not nearly as useful to me right now. I also find the exercises in the HSK Online app helpful.
I tried but disliked this course from Peking University. If your language-learning style works with slower videos, maybe it'll work better for you.
A note on Egyptian: while it's often been claimed that Egyptian wasn't a unique origin of writing, recent evidence (some of it from the venerable Egyptologist who helped teach me at Yale) pushes the timeline back and suggests that it is. Writing within Mesopotamia owes its origin to the city of Sumer, the fourth prominent origin of writing — although there are surely others whose civilizations passed unnoticed by Europeans.