ZC 1.5.0, php 5.6

I have a Finnish language pack installed and Finnish is only language in the shop. We have two months June and July which have a letter ä in them and now I noticed it shows up funny question mark in new products for June in the front page.

Name: language_error.png Views: 130 Size: 743 Bytes

I checked the Finnish language packs locale and checked supported locale from server and settings do match. Also all the other language related pages shows all the umlauts correctly, only this month name shows wrong.

Many years ago the shop had latin-swedish encoding and few years ago it was transferred to utf-8 encoding.

Where should I start to find the error?

Any help would be greatly appreciated with this small but annoying error.