It's not enough and really unfair for them to ask for contribution to all their users, and not open their coordinated data. Just giving some minor exclusive advantages (such as ranks or icons that will render on their own private website for user profiles) is clearly not enough.
We should also campaign against all companies that do such requests to their online communities without really opening the data generated. I am convinced these are abusive practices. They should turn their project to a realy opensource/opendata with commonly accepted licences (attribution is OK, but restrictions such as non-derivatives or non-commercial usage, or required permission for massive reuse, including in competing products, is coimpeltely unfair: they do that only to avoid having to pay regular professional translators and extend their audience rapidly then get more profits and start selling their products at higher prices and create exclusive products that will be sold at expensierve price and based on these translation products);
So yes, Twitter has now created its own translation engine with a large text corpus (harvested from communications by billions users) and now it thinks he no longer needs to thank them.
And we should not just rant against Twitter. We have to ask for this return of cooperative translation efforts (an other mass contribution of data) to Facebook, Google, Bing, which are doing the same, or to various government agencies that want people to contribute to their data without opening them with open data licences ! These have created extremely powerful "'big data" tool that can take control all our life wherever we go and whatever we do. They are too powerful, citizens and competitors MUST be able to reuse this "big data" which is collected. Not "collected", I'd prefer to say really : "stolen" to allow them to steal us more "with our permission" (but do we really have the choice when they have created dependencies, and killed all small competititors using abusively low pricing tactics to create their monopole and a public dependency ?).
Look also at what happened in ISO : open standards now abused everywhere and made mandatory but full of patent restrictions and hidden costs. I rant also against the MPEG LA that have infected the whole IT industry with thir many undisclosed patents when they promoted the adoption of their solutions in worldwide ISO standards.