Concatenate not working in Numbers
Some locales use semicolon as a separator for parameters in formulae, such as Turkey. You can check this by using a comma after the first parameter, then clicking a cell. If you see a semicolon being inserted alongside the comma you should use a semicolon as a delimiter.

                                

This feels like a bug. Copy-pasting the cell shows the syntax error, but copy-pasting the contents of the cell (the formula itself) works fine, alongside me retyping the formula. You can provide feedback to Apple here:


http://www.apple.com/feedback/numbers.html