This sort of thing happens when there are problems with the encoding settings. I've seen it with European languages (other than English) when two or three of these have been set:
Have a look at the system information page to check if the encodings are consistent
The other problem that might cause it is missing fonts on the server (sometimes on the browser, but that usually affects more than just the labels in charts). If the server doesn't have the font Jira is trying to use, it hands it a guess at a replacement, which is often a bad guess and does what you're seeing. To fix that, it's usually a case of installing a font pack on the server, but I'm not sure which one you might want to use for this. I'd have a look at the raw html to see if you can see which font the chart is looking for.
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