I'd prefer if you didn't use Japanese, except for difficult to explain words.
Could you please use English as much as possible and Japanese only when it's necessary?
I'd prefer if you didn't use Japanese, except for difficult to explain words.
説明の難しい単語以外は日本語で言わないでほしいです。
I'd prefer if you used English, except for difficult to explain words.
説明の難しい単語以外は英語で言ってほしいです。
この "I'd prefer" は「~してほしい」より「私の好みは~」に近いですが直接な "I want you to ~" より丁寧です。
次の文も使えます。直接な依頼ですが英語で自然な言い方です:
Could you please use English as much as possible (and Japanese only when it's necessary)?
出来るだけ英語の方で言ってくれますか?(日本語は必要がある時のみに)