What it says on the tin. It can be from the Anglo-Frisian family (Scots, or perhaps an alt-Frisian dialect), or one of the other Germanic branches. The following stipulations are in effect:
-A POD no earlier than 800 AD. The Norman conquest can still be included, but that means the other language must still be able to be understood by speakers of OTL English (and vice-versa),
-Orthography can be different (even radically so), but speakers of this language and English (alternate or as in OTL) must still be able to carry on a conversation,
-English must still be spoken as widely (or at least nearly so) in this TL as IOTL, and
-The alt-language must have at least 5 million speakers worldwide (whether it's only spoken in its country of origin, or as an international trade/imperial language); it can't be a minority language like OTL's Frisian dialects.
Is this doable? Or is it not in the cards with the given POD? Personally I think the Scots language could reach this level of development were the Act of Union not to happen, but are there any other possibilities? Perhaps a prosperous Frisia overtaking the Netherlands in importance on the world stage? Perhaps a surviving Frankish language with similar Gallo-Roman influence that OTL's English received from the Normans?