They should have proper English on that poster since they are a public institution.
I would think their English would be correct given the fact that they are a public agency.
この場合、次のような言い方ができますよ。
ーThey should have proper English on that poster since they are a public institution.
「公的機関なんだから、ポスターに正しい英語が載ってるべきだ」
ーI would think their English would be correct given the fact that they are a public agency.
「公的機関ということを考えると、彼らの英語は正しいと思うよね(でも正しくない)」
given the fact that で「〜ということを考えると・考慮すると」
「公的機関」は public institution や public agency などと言うと良いでしょう。
ご参考まで!