I am not sure whether we can draw a definite conclusion to orcs' possession of fëar on the basis of their [lack of] ability to produce speech and a language. We most likely could if we knew more about 'the' Orcish language itself (as far as that term can be used at all).
If they not only did not have a language of their own (cf. eg. Appendix F), but did not even produce small parts of their langue themselves, one might be able to say with great certainty that this shows the lack of a fëa in these orcs. Even in that case however, it does not totally disprove the existance of some orcs which might have had a fëa nonetheless - they just did not have a (lasting) influence on the Orcish language.
Another point would be whether the relative creation of a language based entirely on already existing languages, in this case, mainly Westron, could already count as the Tolkienian 'talking' and therefore as a sign of a rational soul.
That it cannot serve as proof of the opposite is shown at the example of the Ents. While they ascribe their whole language to the Quendi (being that Entish is a complex system of using apparently solely Elvish roots), creating that system is clearly stated to be their own achievement (cf. Appendix F).
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