オーストラリアは比較的安全だと思っている日本人は多いって英語でなんて言うの？

In saying 'a lot' we are saying, not everyone, but a large number of Japanese people. 'pretty safe' and 'fairly safe' both mean that these people think it is a safe country but admit there may still be crime in the country. On a scale of one to ten it would be about a 8.

Reputation = how something is thought of. A favorable/good reputation meaning thought well of.
Relatively =adverb describing level of safety

"Most and "Many" mean not everyone however a large amount of people. "think and believe" mean how they feel about something, someone or somewhere. "Very safe" means not 100% safe but safe enough that you can travel to Australia without fear.
Example " 90% of Japanese people think that Australia is a safe place to live or visit.

Hope this helps

Thanks

'Generally' is an adverb that may be used when speaking about: the majority, most people, many people, in most cases, or the largest proportion of a group.
EX. "Generally speaking, it's better to prepare all your ingredients before starting to cook."

To share the idea generally means to be of the same opinion. In saying this you are expressing that most Japanese people have the same perception or idea of Australia.When we are speaking generally we are saying that this is the situation in most cases.

"Most Japanese people believe Australia is a safe country." Here is a way to say that Japanese think Australia is a safe country.
"Many people in Japan see Australia as a safe country." See here just means believe, or have an opinion that, Australia is a safe country.

This shows that the opinion Japanese people have of Australia is a good one as there is no danger.