Hope this does not shock you too badly. If anything, it is at least an honest opinion of a native speaker.
Regardless of one's intended meaning and/or nuance, it sounds much more natural to say:
1.「日本語はアメリカ人には難しいと思います。」
than to say:
2.「日本語はアメリカ人に難しいと思います。 」
To be even more honest, my ears do not accept Sentence #2. Sure, I could easily guess what the speaker/author would have wanted to say but I also know without even thinking that Sentence #2 is not something many native speakers would say in a natural setting. We would use 「には」.
Besides 「には」, one could use 「にとっては」, but not just 「に」.
Moving on to the topic of "Americans vs. Others"...
Sentence #1, all by itself without further context, does not mean or imply that people from other countries have an easier time learning Japanese than Americans do. To mean that, it would need to be mentioned in the surrounding sentences.
The easiest way to express that without even creating a whole new sentence would be to say:
「日本語はアメリカ人には特に難しいと思います。」 or
「日本語はアメリカ人にとっては特に難しいと思います。」
「[特]{とく}に」 can be placed right in front of 「アメリカ人」 in either sentence.
Extra:
As an おまけ, here is an example of the "emphatic 「には」 vs. plain 「に」".
「スカイツリーは東京にあります。大阪にはありません。」
I do know that many J-learners would just use 「に」 in the second sentence, too.
