Some time ago I came across with a problem. After I installed Polylang WordPress plugin I have noticed that font has been changed in second language and that change looked unpleasant.
It turned out that this was not a Polylang plugin issue, but rather an incompatible font. To put it short: because of default font incompatibility with the specific language the fallback font was used. In some cases there may be more weird results. For example you may find mixed fonts in single word when fallback font was used only for specific characters that were not present in the default font.
The fix is to use a font fully compatible with a language. The solution is simpler if you can find one font for all languages, this way you only need to change incompatible font and you’re done, otherwise you will need to switch the fonts along with languages.
The specific solution may differ depending on your WordPress setup. You can read how I fixed my blog running “Twenty Sixteen” child theme.