At the beginning there were other, better options but Google Translate was still on top... Like IE on Windows: other were better, but IE was preinstalled...

There are different rules when you use a dominant position in one market to gain share in another.

This is equivalent to the Microsoft/IE thing.

https://en.wikipedia.org/wiki/Competition_law

They're not challenging Google's dominance of search or projecting responsibilities on them as a service due to that monopoly.

They're simply objecting to Google using its dominance in search to gain dominance in, for instance, digital translation.

https://en.wikipedia.org/wiki/United_States_v._Microsoft_Cor....

There are laws about what companies can do. And using a dominant market position in one market to help them in other markets can be an anti-trust violation (see Microsoft and the browser wars).