There is no single right answer to "which language should I learn?" — but there is a wrong one: the language you will not stick with. Three things decide it, and difficulty is only one of them.
1. Why are you learning?
Work, migration, an exam, family, travel or curiosity — the reason sets the method. If it is for a visa, the target is fixed and you study for the exam. If it is curiosity, you will quit the day it stops being enjoyable, so choose a language whose films, music or books you already like.
2. How much contact will you have?
A language you never hear demands far more discipline. If someone around you speaks it, if you enjoy its shows, or if it turns up in your work, progress speeds up on its own.
3. How close is it to your own language?
Related languages move faster, but that alone should not decide it. Someone studying a distant language with real interest outpaces someone grinding a close one without any.
The 13 languages, briefly
**English** — the widest reach: work, academia, the internet. The safe pick if you are unsure. **German** — a route into European work and study; regular grammar, but heavy at the start. **French** — widely used across Europe and Africa; pronunciation is the early hurdle.
**Spanish** — huge number of speakers and forgiving pronunciation; gives a fast sense of progress. **Italian** — spelled as it sounds; ideal if culture or food drew you in. **Portuguese** — Brazil makes it a large market; easy if you already know Spanish.
**Russian** — a wide region; the alphabet takes a week, the cases take much longer. **Arabic** — formal written Arabic and spoken dialects differ, so decide which you want up front. **Persian** — grammar is simpler than people expect; the script is the real threshold.
**Turkish** — regular rules and few exceptions; it accelerates once the suffix logic clicks. **Azerbaijani** — very close to Turkish, the fastest second language for a Turkish speaker. **Japanese** — three writing systems demand patience, but the grammar is less fearsome than its reputation. **Chinese** — simple grammar; characters and tones are what take time.
Still undecided?
If you are torn between two, give each one week. After that week you will notice which one you actually want to open — that answer is more reliable than any ranking.
LexUnio has courses from A1 to C2 in all 13 of these languages, and you can start every one of them free — so you can try before you commit.