"Encountered incorrectly encoded content" error if using acute characters in widget texts at Creator

If I use Creator 3.4.2 on Win7 to make a form and enter national characters into any widget's text, I got "error 1" when compile. Compile output is:
uic: Error in line 1, column 38 : Encountered incorrectly encoded content.
File '..\turbine\mainwindow.ui' is not valid

If I edit all widget texts to standard english letters, it's all fine. I can set non-standard characters later at runtime with ui->lbLabel->setText() succesfully. Only my Creator-generated "mainwindow.ui" has problems if containing accented characters at design time. As much as I know it is a utf-8 coded xml file. I have tried to edit .ui file manually, outside of Creator to replace e.g. "é" character by its utf-8 representation...no success.

This issue is for more versions. I have faced this issue at 5.1, 5.3. and now at Qt 5.5.0 with Creator 3.4.2 as well.
I have Windows 7, Hungarian version. Over english alphabet, we have 16 accented letters in Hungarian. I don't know if you can see these correctly at the forum post, but they are "áéíóöőüű". I don't know if it is the same issue with German, French, Spanish etc "extra" letters...