You are making assumptions. 30% is not a few percent. You are also assuming that merely being on steam and having users encounter your game while browsing will be the primary method people encounter your game and decide to buy it thus the "on a lot less sales" People encounter games via many means. If you only sell it via steam or steam is merely the most convenient means of acquisition then the fact that the buyer bought it on steam isn't proof that they found it via steam or further that they wont buy again if you aren't available via steam.
It is a few percents. All online marketplaces, steam, apple store, airbnb, booking are 20 to 30% commission.
It takes at least 10% commission to be able to run a company of this type. It's possible to run with as low as 5% in some niches, given a very lean operation and a billion dollar scale to amortize fixed costs. Eventually most of the company will be devoted to customer support and billing related tasks.
There is really not that much that can be shaved off. Small studios could never make a platform any efficient. A small group could spend all their time working on distribution and billing, never shipping an actual game. It's a really good deal to just use steam instead.
For AAA games it's annoying to pay 30% and they're rather make their own platform, that's why steam is dropping commission to 20% for top selling hits.
Steam is translated and comes with support for many languages. It takes all currency and it has region settings for pricing, currency and release dates. For the anecdote, it's maybe the only US service that sells well in Russia.
It certainly won't translate the game for you, not that translating a bunch of text files is the hardest part, but it will help with everything else.
Ask international users what they think of other platforms. Like the one they couldn't use because it didn't accept their names with an accent or couldn't enter a billing address without a state.
How will that give your game an international user base? Woudn't the first step being making the game actually playable by users in a given language before worrying about being able to sell to them.
Steam helps you with that. You could try to do better yourself but the effort is not worth saving a few percent of fees (on a lot less sales).