I never seriously investigated the issue, because it was minor, but I found what it was just looking for the reported situations in CIV5Units_Mongol.xml. In the original game files there are certain places where text is written as "<<"", for instance in CIV5GameTextInfos_Espionage_Expansion.xml:
Code:
<Row Tag="TXT_KEY_NOTIFICATION_KILLED_A_SPY">
<Text>Your spy {1_SpyRank} {2_SpyName} killed {TXT_KEY_GRAMMAR_A_AN << {3_CivAdj}} spy in {4_CityName} while they were trying to steal a technology!</Text>
</Row>
Within the newly created CIV5Units_Mongol.xml, it ends up being written instead as <<, since that's probably what ended up within the game database. For instance in the hover text for the unit upgrade:
Code:
<Replace Tag="TXT_KEY_UPGRADE_HELP">
<Text>
Upgrade the unit to {TXT_KEY_GRAMMAR_A_AN << {@1_UnitName}}. This requires {2_Num} [ICON_GOLD] Gold.
</Text>
</Replace>
I have checked, and this issue seems to explain all the text errors that have been reported.
It appears you can correct the issue for now by using ctrl+f and replace to turn all << back into <<. Civ5Units_Mongol.xml is the only file it turns up in. I have not tested this, but I would be very surprised if that didn't fix it.
I'll work on a lua fix, but if you've already made your custom dlc, ctrl+f and replace is much faster.
EDIT: I have tested it. The text errors appear to be fixed.
EDIT2: I've adjusted the lua to search for all "<<" strings and replace them with "<<" before writing, which should correct the text problem. I have not rigorously tested this, so let me know if there are problems.