Translation:It is fun to watch soccer.
I would still stick with "soccer" since that's the english word that the loan word 「サッカー」 is based on, and is the better term in this context of a language learning app. In a different context (sports writing for example) I'd probably agree that one should translate to football or soccer depending on country. I can't think of any context where you would blanket translate to "football" for all english speaking countries.
to chopiniscool: 「マンション」 is a good example of the point I think Fork8 was trying to make, thanks for that. I just don't understand what it has to do with my point regarding the example sentence in question. Obviously it's important to be careful not to directly translate back loan words without considering how the meaning has been changed. In the case of 「サカー」 there has been no change in meaning, so direct translation is appropriate.
Thanks for your advice. And I fond this. http://yamamoto-english.info/to%E4%B8%8D%E5%AE%9A%E8%A9%9E%E3%81%A8%E5%8B%95%E5%90%8D%E8%A9%9E%EF%BC%88%E7%8F%BE%E5%9C%A8%E9%80%B2%E8%A1%8C%E5%BD%A2%E3%80%81%EF%BD%9Eing%EF%BC%89%E3%81%AE%E9%81%95%E3%81%84%E3%81%AB%E3%81%A4/ ただし、it is ～ to ～とか、itが後のto不定詞を指すような構文は基本的に動名詞は使えません。 （たぶん、現在分詞の副詞的用法と区別がつかないから？）
It is important to exercise regularly. good It is important exercising regularly. bad 定期的に運動することが重要です。
He found it difficult for him to enter the college.good He found it difficult his entering the college. bad 彼はその大学に入学するのが難しいことに気がついた。
Rule 3: Infinitives should be used after many adjectives.
Here are three sample sentences that will help to illustrate this rule:
It is not easy to graduate from university.
It is necessary to speak English to work in a hotel.
It is wonderful to have close friends.
When you describe something with an adjective (underlined in the examples above), an infinitive should follow (in bold). Using gerunds here would be incorrect.
I think the course contributors are probably following these kinds of rules in deciding whether to use the gerund, the infinitive, or accept both.
I as a native English speaker think that "it is fun watching soccer" sounds fine, but the native English speaker who commented below disagrees and thinks that it sounds unnatural, so I guess it depends on what kind of English you speak.
As a native speaker of (NZ) English, I find: "I like watching soccer." or "It is fun watching soccer." to be much more natural than "I like to watch soccer." or "It is fun to watch soccer.". I think it's something to do with the continuous nature of the activity. Either "I like taking pictures." or "I like to take pictures." sounds fine, but they mean slightly different things, the first referring to the overall activity, while the second refers to the specific act of taking an individual picture.