ーI didn't really understand what he was saying, but I could tell he was a really good player.
「彼の言っていることはよくわからなかったけど、彼がすごく良いプレーヤーだということはわかった」
I could tell he was a really good player. を使ってゲームスキルが高いことを表現できます。
ーI wasn't familiar with the language he was using, but it was obvious he was quite skilled.
「彼の言語には馴染みがないけど、彼がすごく腕がいいのはすぐにわかった」
スキルという言葉を使うなら、It was obvious he was quite skilled. を使って表現できます。
obvious で「すぐにわかる・明白な」
skilled で「熟練した・腕のいい」
ご参考まで!