Multiple termbases may have different languages: English in one, EN, in the other, Anglais in a third. To make sure that all input that refers to the same language is combined, tick the Merge Languages box. The same language in this case means the language assigned in the field settings.
If you also want to combine language flavours (like EN-US and EN-GB), there's the sublanguages box.
Please refer to the page on Merging Languages for more details.
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