The intent of Condition #5 of the Open Font License seems to simply be to clarify the following:
- You do need to provide this license alongside your usage of the font, but
- This license applies just to the font, and doesn't affect whatever you're using the font to produce.
In other words, the license is clarifying that it is not a viral license such as the GPL.
It's written assuming you're creating something classical like a written document since that would be the most ordinary use case. However in a non-classical production like a game, we could reasonably interpret that the “document” just covers wherever you're using the font inside the gam. It may even be interpreted as the game itself! But since the point of the clause is to clarify that it's not affecting any of that, how much is counted as the “document” would be mostly academic.
So it should be safe to use these fonts in a game — they're not exclusively limited to plain documents.
Proper attribution is specified per condition 2 of the Open Font License:
2) Original or Modified Versions of the Font Software may be bundled, redistributed and/or sold with any software, provided that each copy contains the above copyright notice and this license. These can be included either as stand-alone text files, human-readable headers or in the appropriate machine-readable metadata fields within text or binary files as long as those fields can be easily viewed by the user.
This gives you plenty of room for how to provide attribution. There's two main options games go for:
- If your users can access the game's files (e.g. it's not a console game), then you can distribute the license as a plain text file within your game files. If you've just got one or two OFL fonts, you might include it as
foo-license.txt where “foo” is the font name. If you have many OFL fonts, just include one
open-font-license.txt, and insert some lines at the top clarifying that the license text that follows applies to a specific set of fonts used within your game (and list the names of those fonts).
- Alternately, many games include a Boring Legal Stuff section. This section is stuffed somewhere users won't find it unless they're looking for it, such as a the end of the About or Help area. It generally lists all the licenses involved in the game's production in a single endlessly scrolling text box, since that's technically sufficient to satisfy licensing requirements. You can reproduce the OFL license text in here and indicate which OFL fonts you've used that it covers.
Before releasing your game to the public for sale, it would be standard procedure to visit a lawyer to make sure you're doing things safely and legally. It would be appropriate to check with that lawyer that the above is sufficient just to cross your t's and dot your i's.