In the example above, the charts series label (and therefore also the chart legend) is translated using page item P1063_LANG_AVERAGE_SAL.
Note that in the chart series SQL, make sure to add a space after the column alias like this: "&P1063_LANG_AVERAGE_SAL. ". Otherwise you'll get an parsing error because Oracle thinks the label is null.
The x-axis is formatted in the chart series query using page item P1063_NLS_DATE_LANGUAGE.
The y-axis and chart labels are formatted in the chart xml using page items P1063_NLS_NUMERIC_CHARACTER_G and P1063_NLS_NUMERIC_CHARACTER_D. Note the use of the backslash in the set_page_items process, needed to escape the comma character. You need to use Custom XML for the chart, and change the <format> tag as shown in the code below.
You'll probably want to use FSP_LANGUAGE_PREFERENCE and apex_lang.lang for these translations in real applications, I just used a select list and some page items as a quick example.