"We will go on foot"
Translation:Tutaenda kwa miguu
I think a more pertinent question would be why we use the singular in English ... because going on foot almost always involves more than one foot. "By feet" or "with legs" (another way you could literally translate it) makes more logical sense ... unless you're literally hopping. Languages are not always completely logical. They're spoken by people who copy from other people and it becomes habit long before it's questioned. Idiomatic use often has a whole lot of funny phrases that, when you think about them, are actually quite strange, but if you said it the more logical way, would sound strange or even be regarded as incorrect,
For example, why do we say "I'm interested in dinosaurs" and not "about dinosaurs" like we do most other times we're introducing a topic or theme of something? I'm not in dinosaurs!! :-P It's just convention.