I briefly moved some of my fixed income into foreign bonds, but unwound it because I could find no studies that indicate foreign fixed income would improve my overall portfolio performance. I tend to use Lawrence Swedroe's guidance. He advises a pretty heavy allocation to foreign equities - but not foreign bonds.
Your fixed income allocation should be an anchor providing lower volatility and a source of income/cash so that you do not need to liquidate equities at the wrong time. On that theory, anything that is more complicated/risky than domestic fixed income is not serving this goal and should not be considered part of your fixed income allocation.
You will definitely have increased risk due to exchange rates, which can hedge against a falling dollar - but a foreign equity allocation will do the same and hopefully provide a greater return to compensate you for your risk.