Latin languagesystem breaks variable font build

It took me a long time to narrow it down, but I'm hitting an issue where having a Latin language system declaration in the features for a variable font causes the process for generating with fontmake to fail.

For example, if the features start with just the default language system, the font generates fine, as expected:
```
languagesystem DFLT dflt;
```

But if I have the default plus Latin, like so:
```
languagesystem DFLT dflt;
languagesystem latn dflt;
```

… then fontmake chokes and throws the following tracebacks (which are a bit cryptic):
```
Traceback (most recent call last):
File "/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/site-packages/fontmake/font_project.py", line 1127, in run_from_designspace
self._run_from_designspace_interpolatable(
File "/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/site-packages/fontmake/font_project.py", line 1223, in _run_from_designspace_interpolatable
self.build_variable_fonts(
File "/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/site-packages/fontmake/font_project.py", line 420, in build_variable_fonts
fonts = ufo2ft.compileVariableTTFs(
^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/site-packages/ufo2ft/__init__.py", line 633, in compileVariableTTFs
vfNameToTTFont = varLib.build_many(
^^^^^^^^^^^^^^^^^^
File "/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/site-packages/fontTools/varLib/__init__.py", line 1012, in build_many
vf = build(
^^^^^^
File "/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/site-packages/fontTools/varLib/__init__.py", line 1092, in build
_merge_OTL(vf, model, master_fonts, axisTags)
File "/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/site-packages/fontTools/varLib/__init__.py", line 700, in _merge_OTL
merger.mergeTables(font, master_fonts, ["GSUB", "GDEF", "GPOS"])
File "/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/site-packages/fontTools/varLib/merger.py", line 157, in mergeTables
self.mergeThings(font[tag], [m.get(tag) for m in master_ttfs])
File "/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/site-packages/fontTools/varLib/merger.py", line 1206, in mergeThings
super(VariationMerger, self).mergeThings(out, lst)
File "/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/site-packages/fontTools/varLib/merger.py", line 144, in mergeThings
self.mergeObjects(out, lst)
File "/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/site-packages/fontTools/varLib/merger.py", line 115, in mergeObjects
mergerFunc(self, value, values)
File "/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/site-packages/fontTools/varLib/merger.py", line 1206, in mergeThings
super(VariationMerger, self).mergeThings(out, lst)
File "/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/site-packages/fontTools/varLib/merger.py", line 144, in mergeThings
self.mergeObjects(out, lst)
File "/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/site-packages/fontTools/varLib/merger.py", line 115, in mergeObjects
mergerFunc(self, value, values)
File "/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/site-packages/fontTools/varLib/merger.py", line 1206, in mergeThings
super(VariationMerger, self).mergeThings(out, lst)
File "/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/site-packages/fontTools/varLib/merger.py", line 144, in mergeThings
self.mergeObjects(out, lst)
File "/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/site-packages/fontTools/varLib/merger.py", line 115, in mergeObjects
mergerFunc(self, value, values)
File "/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/site-packages/fontTools/varLib/merger.py", line 1206, in mergeThings
super(VariationMerger, self).mergeThings(out, lst)
File "/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/site-packages/fontTools/varLib/merger.py", line 146, in mergeThings
self.mergeLists(out, lst)
File "/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/site-packages/fontTools/varLib/merger.py", line 125, in mergeLists
self.mergeThings(value, values)
File "/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/site-packages/fontTools/varLib/merger.py", line 1206, in mergeThings
super(VariationMerger, self).mergeThings(out, lst)
File "/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/site-packages/fontTools/varLib/merger.py", line 144, in mergeThings
self.mergeObjects(out, lst)
File "/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/site-packages/fontTools/varLib/merger.py", line 115, in mergeObjects
mergerFunc(self, value, values)
File "/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/site-packages/fontTools/varLib/merger.py", line 1206, in mergeThings
super(VariationMerger, self).mergeThings(out, lst)
File "/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/site-packages/fontTools/varLib/merger.py", line 144, in mergeThings
self.mergeObjects(out, lst)
File "/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/site-packages/fontTools/varLib/merger.py", line 115, in mergeObjects
mergerFunc(self, value, values)
File "/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/site-packages/fontTools/varLib/merger.py", line 1206, in mergeThings
super(VariationMerger, self).mergeThings(out, lst)
File "/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/site-packages/fontTools/varLib/merger.py", line 144, in mergeThings
self.mergeObjects(out, lst)
File "/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/site-packages/fontTools/varLib/merger.py", line 115, in mergeObjects
mergerFunc(self, value, values)
File "/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/site-packages/fontTools/varLib/merger.py", line 1206, in mergeThings
super(VariationMerger, self).mergeThings(out, lst)
File "/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/site-packages/fontTools/varLib/merger.py", line 149, in mergeThings
raise ShouldBeConstant(self, expected=out, got=lst)
fontTools.varLib.errors.ShouldBeConstant:

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
File "/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/site-packages/fontmake/__main__.py", line 675, in main
project.run_from_designspace(inputs.designspace_path, **args)
File "/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/site-packages/fontmake/font_project.py", line 1142, in run_from_designspace
raise FontmakeError(
fontmake.errors.FontmakeError:

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "/Library/Frameworks/Python.framework/Versions/3.11/bin/fontmake", line 8, in
sys.exit(main())
^^^^^^
File "/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/site-packages/fontmake/__main__.py", line 700, in main
sys.exit(f"fontmake: Error: {str(e)}")
^^^^^^
File "/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/site-packages/fontmake/errors.py", line 33, in __str__
cause = str(self.__cause__) if self.__cause__ is not None else None
^^^^^^^^^^^^^^^^^^^
File "/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/site-packages/fontTools/varLib/errors.py", line 70, in __str__
return "\n\n" + basic + location + self.details
^^^^^^^^^^^^
File "/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/site-packages/fontTools/varLib/errors.py", line 86, in details
good_ttf = next(
^^^^^
StopIteration
```

Changing the Latin declaration to something else like Cyrillic or Arabic results in a different error that may be related and/or may be triggered by an issue with some inconsistency between my sources:
```
fontmake: Error: In 'Fubar.designspace': Generating fonts from Designspace failed:

Couldn't merge the fonts, because some values were different, but should have
been the same. This happened while performing the following operation:
GPOS.table.ScriptList.ScriptCount

The problem is likely to be in Fubar Condensed Regular:
Expected to see .ScriptCount==3, instead saw 2
```

For what it's worth, there is only one .fea file being used for all features and language declarations in the variable font.