Bi-monthly means once every two months.

But it also means twice a month.

Either is acceptable. The safest usage is to throw in semi-monthly if we mean twice a month. Otherwise, this becomes quite confusing.

The same rules apply to bi-weekly (and semi-weekly).

Biannual and biannually are treated in the same manner. Don't forget semi-annual and semi-annually).

But biennial has somehow acquired the singular meaning of every two years.

We have cognates. We have words spelled the identical way with several diffferent meanings. We have slang. We have words that sound alike but have different spellings. We have different pronunciations of the same word based on geography. We have different spellings of the same word based on geography. Finally, all of this is in a state of constant flux.

I was in a spelling bee in the sixth grade. My word (?) was "all right". My thinking was that a spelling bee should present one word, not two in the contest, though I knew that "all right" was correct. I answered "alright" since that was one word.and I was confused by this. I had to sit down. Now, many years later, "alright" has gained acceptability.

Kudos to all who have adopted English as a second or third language.