[slowsmile]

@Becky...Regarding just getting the line number for the meta language identifier, you could perhaps put Kevin's code into a simple function and get the line number something like this:

def getOPFLineNumber(opf, search_text):
    opf_data = opf.splitlines()     # split the opf string into separate lines of code
    
    # assign a line num to the search text when found
    linenum = ''
    for index, line in enumerate(opf_data):
        if search_text in line:
            linum = index + 1
            linenum = str(linum)
            return(linenum)

Then your plugin.py code could perhaps be adjusted to look like this:

  dc_language = None
    meta_language = tree.find('.//{http://purl.org/dc/elements/1.1/}language')
    if hasattr(meta_language, 'text'):
        dc_language = meta_language.text

    if dc_language:
        if dc_language != "pl":
            opf_data = bk.get_opf()    # get all the opf data as a string
            linenumber = getOPFLineNumber(opf_data, "<dc:language>")
            message = "Language specified in metadata is other than 'pl' (Polish)"
            bk.add_extended_result("error", "content.opf", linenumber , 0, 'Becky Metadata #002' + ' -- ' + message)
    else:
        message = "Language not specified in metadata"
        bk.add_extended_result("error", "content.opf", 0, 0, 'Becky Metadata #001' + ' -- ' + message)

I've used a similar getOPFLineNumber() function in my IDErrorCheck plugin which works without problems to get OPF line numbers on error.

Also, I know that using bk.get_opf() will certainly work for an Edit plugin but I'm not at all sure whether this bk method will be available for a Validation plugin like your plugin. If this is true then you could probably use xml.etree to get the opf file contents as a string instead of using bk.get_opf().