Can commlinks do that, too?
Game mechanics wise? No.
It seems like they out to be able to.
I agree; in SR4, all it took was a commlink with a sim module.
And if they do, what are the use of linguisofts?
Linguasofts are (and were, in SR4) the programs that did the translation. In SR4, these programs ran on a commlink with a sim module; in SR5, they require a skilljack.
Especially if we're talking about an implanted commlink and DNI, what makes linguisofts useful?
In SR4, Linguasofts were easily accessible means for characters to overcome language barriers. In SR5, they require a minimum investment of 20k and 0.1 Essence for a Rating 1 Skilljack, plus at least 1k per Linguasoft.
and if commlinks can't do that, is there a good, in game reason i can use to explain why not to my players?
Is there a good, in-game reason? You'll have to decide that for yourself.
Keep in mind that while a smartphone app may be able to translate speech to text (or even speech to speech), we're still FAR away from Babelfish-like automatic translators; nuances of most languages make programming a machine to understand subtle differences incredibly difficult; a linguasoft essentially provides your brain with inherent understanding of a language, enabling you to both understand and make yourself understood as if you had learned the language from birth. A smartphone app would be the equivalent of having someone translating for you, but worse.