Translation:We practised for two months.
There are differences in spelling between American and British. "Practiced" and "practised" are both correct.
I am American, but i favor "practise" because it is more fonetic. I am a firm supporter of (ov) English spelling reform; "foneticizing" as i call it; given that millions of English-speakers ar functionally illiterate, no-one (-wun) should be able to, in good conscience, choose the clearly-less-fonetic ov two or more accepted spellings.
Using "have" means that the two months of practicing were the last two months (up to the present). The Chinese sentence does not specify when the practicing happened, just that it lasted two months.
This would be written differently in Chinese, though.
我们练习了两个月。We practiced for two months.
我们练习了两个月了。We have practiced for two months.
Actually, Chinese doesn't have tense as a grammatical category, but it does have aspect (perfect and imperfect)--so in fact both sentences can be translated as "have practiced" (perfect/completed aspect). It is true that present perfect tends to describe events that have occurred, and hence these can be framed as past tense (i.e. in languages that have tense).
The second sentence, where there is a sentence-final 了, can be interpreted as conveying other information, such as change-of-state.
practised?????????!!!!!!!!!!???????!!!!!!!!! The correct spelling is practiced!!