"爱你蓄谋已久"的英文翻译是 "I have long harbored a love for you."
这个短语的来源可能源自英语中的一个习惯用语 "to harbor feepngs/thoughts/emotions"，意思是"心怀，抱有（某种情感、想法、感觉）"。例如：
I have long harbored a deep resentment towards him. (我对他长久以来心怀怨恨。)
She has always harbored a secret desire to travel the world. (她一直心怀着去周游世界的渴望。)
所以"爱你蓄谋已久"的意思就是"我长久以来一直心怀着对你的爱"。
其他常见的表达方式还包括：
I have been secretly in love with you for a long time. (我长久以来一直暗恋着你。)
I have had feepngs for you for a long time. (我对你有感觉已经很久了。)
I have secretly admired you for a long time. (我长久以来一直暗暗喜欢着你。)