$.tablesorter does a lot of guesswork trying to infer what does the content in a table mean. It sometimes work, mostly for English; however, trying to (for example) guess if a column contains dates just plain doesn't work for some, if not most, languages.
Apart from numbers, $.tablesorter tries to guess dates and times in four distinct formats, currencies, IP addresses, and for some reasons URLs.
I'd kill everything but detecting and sorting numeric values, and even this I'd do differently (using "natural sorting" - identifying runs of digits and non-digits and sorting them separately). [http://www.codinghorror.com/blog/2007/12/sorting-for-humans-natural-sort-order.html]
For sorting any remotely complicated data one should use a data-sort-value attribute on the <td> (which will be used as the sortkey for given cell), or a template that wraps it.
Bugs probably caused by or related to this misfeature (I didn't double-check): {T17404}, {T33137}, {T36475}, {T41069}, {T44097}, {T44607}, {T44924}, {T45165}.