I've heard Americans use biweekly to mean every two weeks. I've also seen them use it to mean twice a week, but more rarely than the other meaning.
In the UK and other English speaking areas (Australia and New Zealand to name two) bi-weekely always means twice a week, following the same pattern as biannually and other examples. We use fortnightly to mean every two weeks, but the word fortnight is not commonly used in the US.
Bimonthly is also ambiguous unfortunately. Again in the UK it would be taken as meaning twice per month (almost, but not quite, the same as fortnightly) but some others use it to mean every two months.
Biannually seems to be consistently used and understood to mean twice per year, with biennial meaning every two years though that isn't a commonly used word in my experience (people tend to just say "every two years").
One (relatively) common usage of biennial (or its Italian spelling biennale) is in the art and cinema community as competitions tend to happen every other year and are called a biennale even in English
You mean to say "semiannually", of course. As I've been told by many people for whom English is a second language, no one speaks English worse than the English. b^)
In the UK and other English speaking areas (Australia and New Zealand to name two) bi-weekely always means twice a week, following the same pattern as biannually and other examples. We use fortnightly to mean every two weeks, but the word fortnight is not commonly used in the US.
Bimonthly is also ambiguous unfortunately. Again in the UK it would be taken as meaning twice per month (almost, but not quite, the same as fortnightly) but some others use it to mean every two months.
Biannually seems to be consistently used and understood to mean twice per year, with biennial meaning every two years though that isn't a commonly used word in my experience (people tend to just say "every two years").