There is a self-contained example at the end of this answer, but here are your two problems:
Large Numbers
The best way to fix this is to use 'excel' instead of 'csv' here:
dom: 'Bfrtip',
"buttons": [
'excel'
]
This will ensure the Excel cell format is "number" instead of "general".
I don't know of a way to automatically control the Excel cell format when using the CSV export option - unless you are prepared to save the CSV as a text file, then import into Excel and format it during the import (a manual process).
Accented Characters
There are various reasons why you could be having this issue - many of which are outside the scope of DataTables - so the following may not help you, but...
Make sure your HTML page contains this inside the head
tag:
<meta charset="UTF-8">
This is sufficient for me to get my demo working (see below). For example:
However, like I say, there could be many other reasons - for example, see here.
Full Example
Paste the following HTML into a text file (use Notepad++ not Notepad, if you are on Windows). Assuming Notepad++, make sure the file is saved as UTF-8 - menu > Encoding > UTF-8. Then open the file in any browser.
You don't need all of those JS imports provided below (for example the PDF one); feel free to remove extra ones. (I have them for a fuller demo and was too lazy to remove them.)
<!doctype html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Export to Excel</title>
<script src="https://code.jquery.com/jquery-3.3.1.min.js"></script>
<script src="https://cdn.datatables.net/1.10.20/js/jquery.dataTables.min.js"></script>
<link rel="stylesheet" type="text/css" href="https://cdn.datatables.net/1.10.20/css/jquery.dataTables.min.css">
<link rel="stylesheet" type="text/css" href="https://datatables.net/media/css/site-examples.css">
<!-- buttons -->
<link rel="stylesheet" type="text/css" href="https://cdn.datatables.net/buttons/1.6.1/css/buttons.dataTables.min.css">
<script src="https://code.jquery.com/jquery-3.3.1.js"></script>
<script src="https://cdn.datatables.net/1.10.20/js/jquery.dataTables.min.js"></script>
<script src="https://cdn.datatables.net/buttons/1.6.1/js/dataTables.buttons.min.js"></script>
<script src="https://cdn.datatables.net/buttons/1.6.1/js/buttons.flash.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/jszip/3.1.3/jszip.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/pdfmake/0.1.53/pdfmake.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/pdfmake/0.1.53/vfs_fonts.js"></script>
<script src="https://cdn.datatables.net/buttons/1.6.1/js/buttons.html5.min.js"></script>
<script src="https://cdn.datatables.net/buttons/1.6.1/js/buttons.print.min.js"></script>
</head>
<body>
<div style="margin: 20px;">
<table id="example" class="display nowrap dataTable cell-border" style="width:100%">
<thead>
<tr>
<th>Name</th>
<th>Position</th>
<th>Office</th>
<th>Age</th>
<th>Start date</th>
<th>Salary</th>
</tr>
</thead>
<tbody>
<tr>
<td>Adélaïde Nixon</td>
<td>System Architect</td>
<td>Edinburgh</td>
<td>6123456789012345</td>
<td>2011/04/25</td>
<td>$320,800</td>
</tr>
</tbody>
<tfoot>
<tr>
<th>Name</th>
<th>Position</th>
<th>Office</th>
<th>Age</th>
<th>Start date</th>
<th>Salary</th>
</tr>
</tfoot>
</table>
</div>
<script type="text/javascript">
$(document).ready(function() {
$('#example').DataTable({
dom: 'Bfrtip',
"buttons": [
'excel'
]
});
});
</script>
</body>
Note on the CSV Option
If you do use "csv" instead of "excel" in your button definition, and if you open the resulting file in a text editor, instead of Excel, you will see this data:
"Name","Position","Office","Age","Start date","Salary"
"Adélaïde Nixon","System Architect","Edinburgh","6123456789012345","2011/04/25","$320,800"
The data is the way you need it to be - it's just that Excel will make various assumptions about how to format the data when opening the csv file.