I'm wanting to create a bilingual social media account. I'm not sure how to say "Japanese below" (as in, under the first part which will be written in English) in a grammatically correct way. 「日本語以下」makes the most sense to me, but I'm not sure if that would the correct way to say 'Japanese to follow' in this context. Any insight would be appreciated, thank you.
It was close, but the word order is wrong. "日本語以下" means something like "below Japanese" or "worse than Japanese."
Instead, you can say "以下日本語". For readability, you may want to add a comma after 以下 ("以下、日本語"). Alternatively you can say "ここから日本語" (lit. "From here, (in) Japanese").
Note that these work only between the English part and the Japanese part, because 以下 refers to something right after it. If you want to say a similar thing before the English part , you can say something like "日本語版は下にあります" (lit. "There is a Japanese version below").