Workflow-wise I find myself editing the terms (key names) and language text via either the Google spreadsheet or even by editing the I2Languages.prefab (YAML) file directly using a text-editor.
It tends to be faster to rename or change text than going through the LanguageSource inspector. This is mainly because it requires a lot of clicks and thinking to find the terms you want because the filter menu is clumsy and the search is limited. Here are some interface improvements that would really help:
99% of the time I just want to see a single term group. So I have to click Nothing and then click the group I want. I think you should be able to hold down SHIFT to it show only this group, no matter what the state of the filter is at the moment. Or perhaps a better idea would be to reverse that and have SHIFT add or remove from the current selection and without SHIFT replaces it.
Currently the Search field under the terms UI doesn't search for group names. I often use groups and then the final term key is just a number 0,1,2,3,4, etc. That way I can use the very handy GetTermsList("Game/Praise/Good") to enumerate possible messages. So for these terms I need to be able to search by the group name. I have a lot of groups.
Similar to #2, I'd like to be able to search for language text. For example, there's a lot of power in being able to find all instances of a certain word. For example if I want to replace "color" (en-US) with "colour" (en-GB) without having to find them in the game UI or across multiple tabs in a spreadsheet.
I can't think of a reason not to just have the term search find everything (terms, text, and groups). Since you can further filter by group, it's easy to narrow down the results pretty quickly. Maybe other people have an opinion on this?