わからない単語をそのまま英語にすると「Words I don't know」になります。
わからない事は「Don't know」です。
「分からない単語を調べながら話した」と言うなら、「I was speaking while checking for words I didn't know」。
過去形なので didn't。
調べる checking for！
There were words I had to look up while we were talking because I didn't know how to say them.
There were words I didn't know so I looked them up while we were chatting.
1) There were words I had to look up while we were talking because I didn't know how to say them.
「何というか分からなかったので、話しながら単語を調べなければいけなかった。」
2) There were words I didn't know so I looked them up while we were chatting.
「知らない単語があったので話しながら調べた。」
「分からない単語」は words I didn't know と言えます。
「調べる」は look up
ご参考になれば幸いです！
イギリス 
日本 
