You don’t achieve tech. You either achieve/reach a tech level, or you develop tech.
There’s a dropped letter in the second sentence. Did you spot it?
Some people get confused about which indefinite article (a/an) to use. Some teachers tell their students, “Use a with words that start with a consonant and an with words that start with a vowel,” but that falls apart as soon as the kids encounter unicorns. 🙂 (Unicorns is pronounced as if it started with a y: yoo-ni-corns. On the other hand, the h in a word such as honor is silent, so you’d say an honor instead of a honor, even though it begins with a consonant.)
I changed the position of finally to improve the flow of the sentence.
Humanity has finally developed the tech to leave our solar system. When they arrive in a nearby system, they find an inhabited planet broadcasting in fluent German.
Whoever started this trend for referring to any group of humans as humanity, as if any thing done by some humans is done by all humans collectively, needs to stop. Humanity didn’t arrive at a nearby system; humans did.
Try this instead: Humans finally developed the tech to leave their solar system. When their ship arrived in another system nearby, they found an inhabited planet broadcasting in fluent German.
Now you probably want to know why I changed the first part of the second sentence… For one thing, it ended with system, just like the previous sentence, and that seemed clunky. Also, you want a “lift” to the movement of the sentence there, and the original version had a falling/final-statement feel instead. (Sorry. The synesthesia is acting up a lot today, so I’m thinking of sentence flow/movement/shape/texture in a slightly literal way. That sentence really did feel as if it were dropping where it ought to rise a little… so I fixed it.)
You may be amused to know that WordPress’ spell check isn’t recognizing humans as a real word today.