We distribute "Aleo", but this font does not include cyrillic characters or some other extended latin characters (like "ğ" and "Ğ", from Turkish). It does include some extended latin, like the Polish "ł".

Lato does include these characters, so header text which mixes latin languages renders in a mixture of Aleo and Lato.

For most languages, this doesn't seem to be a particularly big deal. For example, here's Turkish with the fallback set to "Impact" to exaggerate differences:

However, the actual fallback (to Lato) isn't visually distinguishable to me, particularly in this case since both Aleo and Lato have similar (identical?) shapes for the "g" glyph:

When the mixed language is a cyrillic language, we get this instead: cyrillic in Lato, basic latin in Aleo.

Here's the entire text in Lato instead:

Some things we could do:

• Nothing, since this only creates a minor subjective problem in an edge case (mixing Cyrillic and standard Latin glyphs in an object title).
  • This problem may be more serious for a language which uses a mixture of basic latin and extended latin glyphs, but I couldn't immediately find one: Polish seems to have the glyphs, Turkish doesn't seem to have distinguishable glyphs, and Russian doesn't (normally) use basic latin glyphs.
• Stop distributing Aleo. Use Lato, not Aleo, for headers.
• Find some other header font with more glyphs. Use that, not Aleo, for headers.
• On the server, detect that an object title has only basic latin glyphs, and emit a CSS class like "use-aleo" if it does to render it in Aleo. If the class is not present, use Lato. This will mean that adding a cyrillic character to an English title will remove serifs, but this is probably less jarring than having both styles adjacent in the same header.
• Find or create a copy of Aleo with these missing glyphs (perhaps by sponsoring the work from @AlessioLaiso).