What would happen if everyone in the world suddenly spoke the same language?
Here are several possible results if everyone in the world suddenly spoke the same language:

- Easier and more accurate communication. Besides no longer needing laborious translation, some information that is difficult to translate accurately due to different habits and other reasons can be understood more accurately.
- New products and standards. If the new world language is quite different from what we have, many products related to languages would need to change, such as keyboards, signs, and operation systems. Many standards such as official language and character encoding standards would also need an update.
- Information spreads faster. Though we have translation software that can process images and a whole webpage, their limited performance still stops many people from visiting foreign websites and media. The new world language would allow people to access more information sources.
- Better understanding of cultures. Languages are not only tools but also reflections of different cultures. If these culture-related parts can be fully transferred into the new world language, they can be easier understood by others.
- Possible controversies. With the sudden spread of a language, many might fear that the convenience would make the speakers' traditions and cultures connected to their original languages slowly fade away. It might also spark squabbles around whose language is more similar to the new world language and therefore superior/simpler.
I think it would be a terrible loss if all the other languages were eliminated.
You are right. When we lose a language, we also lose a lot of related histories, cultures, local information and knowledge, etc. For example, information about local species and climate patterns, historical events, and spiritual attitudes of speakers. This can also cause health problems among the original speakers, as their identity, autonomy, and spiritual sovereignty are usually highly connected to their traditional language, and the loss can lead to mental problems such as trauma and depression.
Even if the new world language doesn't immediately eliminate other languages, many of them are still likely to be lost. The gradual abandonment of a native language in favor of a bridge language, which allows people with different native languages to communicate, has already been a typical reason for language death in the modern period, and this can happen even faster when we suddenly get a world language.