For example, if you're on the Latin America North server and you wish to stream on twitch, to a predominantly English audience, and you plan to speak English on stream, you should be able to go to Settings and select English for your static text language. This wouldn't affect the language of any "live" or regional content.
I fully understand why it would be difficult to translate all the news, and youtube videos into every language, while still maintaining their regional targeting. For example, in Mexico, the promo videos are not just Spanish, but they sometimes target Mexico specifically, so they'll talk about Mexican teams, or events that are happening only in Mexico. Translating that into every language would be unreasonable, and that's not what I'm asking for in this topic.
I'm asking that after you select your region and language, and you log into the client, you should be able to then go to Settings and select any language you want for the static text, such as the User Interface (things like buttons), patch notes, tool tips, item and rune descriptions, announcers (in-game), etc.
Players will still speak Spanish, player names may still use keyboard characters or fonts that are language-specific to your region, and any live content will still be Spanish. Likewise, if you attempt to type anything into the chat, it will still use the font that is specific to your region (for example, Korean characters if you are in Korea).
Thank you Riot for considering this. It would really help those of us who want to stream on twitch, and who would like to target an audience that is different than the region we are playing in.