翻译 - 巴西葡萄牙语-拉丁语 - Nunca desista de seus sonhos

Both versions (with conjunctive and with imperative) are pretty good, niorik. Just imperative form is more stronger and expresses rather the order, while conjunctive - rather the request.
Anyway, bravo!

Thanks, Aneta.
But I'm still a bit worried about that. I just don't know if nunquam makes the sentence negative - which means you cannot use the imperative mood, and has to substitute with a conjunctive or another formation (like noli + infinitive). I tried looking for Classical Latin examples of this use, but I couldn't find any - only Medieval and Modern Latin usage, which is obviously based on the writer's own mother tongue as often as not.

Yes,the imperative version should sound rather: "noli umquam relinquere".