At enwiki, the following gives "2.0E-5" (correct) but at bnwiki it gives "2.0" (missing exponent).
mw.getContentLanguage():formatNum(0.00002)

Links to test modules follow, with results on their talk pages.

enwiki module and output.

bnwiki module and output.

In another test (not saved), using mw.getContentLanguage('bn') at enwiki gave the correct enwiki result, although so did mw.getContentLanguage('xyz') so I guess it was falling back to en.