The opposite of selfishness is allegedly self-sacrifice, but self-sacrifice is nothing but enabling the most selfish greed.
When a mother sacrifices her life for her child, that's not self-sacrifice. That's just a good trade; she believes her life is worth less than the kid's life. Likewise, you believe the money is worth less than the food you buy at the grocery store, which is why you make that trade. Nobody calls a trip to the mall 'self-sacrifice' and they're extremely right about that.
Self-sacrifice is actively Satanic. It selectively gets more-virtuous folk killed, and leaves alive the degenerates. If someone tells you to be self-sacrificing, the correct answer is always, "Oh, you first. Please, show us all how it's done. No no, I insist." Make sure they're "sacrificed" immediately if not sooner. Absolutely never sacrifice yourself.
Refresher: psychological egoism is true, so selflessness isn't merely dubious, but logically impossible. Simply embrace your selfishness, because trying to reject selfishness is to rebel against Gnon. By definition, Gnon always wins.
In general, "sacrifice" seems to mean [cost, but, like, big]. In particular I want to call out permanent sacrifices. Let's start with impermanent sacrifices (except insofar as time costs are always permanent; this should go without saying, but doesn't). I "sacrifice" money at the holy temple [grocery store], and having sacrificed fittingly, I receive the blessing [18% cream] and [ground meat]. Praise be to edibilia. Let us consume extropy so as to emit glorious entropy.
By contrast, if I propose to sacrifice my little finger for groceries, I draw forth much skepticism. Why should I make myself permanently weaker? Unnecessary at best.
Did Odin have to give up an eye to learn runes? Nope. Could have remained whole, but added runes to that wholeness. He dun fukd up. What selfish, envious dickface took his eye? It's not even useful to non-Odin beings; it will just die once separated from him.
Should you "sacrifice" for your goals? You should make good trades. Accumulate wealth rather than squandering it. In theory, sure if you're gaining a permanent boon from a permanent sacrifice, there can be an argument. If that power is necessary for your goals and there's no non-permanent cost, then yeah it's a good trade and you should make it. However, in real life, I have never seen an example of such a thing. In reality, even if you accumulate permanent boons you run out of permanent things to sacrifice and become paralyzed.
Even though time is always a permanent sacrifice, it is also an endless fountain of wealth. As long as you exist, there's more time where that came from. Cut off as much time you want; it will regenerate.
On the contrary, the best trades are about making permanent improvements in exchange for temporary costs. That is, empowering your body. Be yourself, but moreso. Make yourself stronger, and achieve your goals (e.g. learning a language) as a side effect of getting or being stronger.