Interesting! But what about descriptions? To me it seems as if descriptions should be doable in both languages.
We constantly use both Japanese and English in title, description and tags. No problem at all. The algorithm will match your different tags toward the two different audiences and see which one has higher audience engagement metrics. In anyway - do not use different languages just to generate traffic (to trick the algorithm) but if you decide for example to make a movie about your Spanish language studies progress - there is no problem to include some Spanish tags too.
Also you can add/upload subtitles to your videos in any language you desire - there are no limitations here.
For plurals I do not know but I noticed that they can influence significantly your position in the search results especially if they are in the title. Basically use Google trends or AdWords -->Tools--> Keywords planner to find which are the most "hot/popular" and relevant to your channel and interests keywords and keywords
combinations/phrases. For the last one you'll need an AdWords account so make one. (Just one warning - do not become a "slave" of keywords and hot topic searches - use the insights you gain with discretion!
)
Also in the description the first 1 or two sentences are the most important one. You may wish to make them more interesting and relevant to the topic of the video but it is up to you.
Just wanted to add at the end - Good Luck with your channel!