They appear when building:
//chrome/android:chrome_java

E.g.:
../../../../../../tmp/tmpd3ZOHh/r5/values-v21/restriction_values.xml:295 "`VariationsRestrictParameterTitle`" is not translated in "am" (Amharic), "ar" (Arabic), "bg" (Bulgarian), "ca" (Catalan), "cs" (Czech), "da" (Danish), "de" (German), "el" (Greek), "es" (Spanish), "es-US" (Spanish: United States), "fa" (Persian), "fi" (Finnish), "fr" (French), "hi" (Hindi), "hr" (Croatian), "hu" (Hungarian), "in" (Indonesian), "it" (Italian), "iw" (Hebrew), "ja" (Japanese), "ko" (Korean), "lt" (Lithuanian), "lv" (Latvian), "nb" (Norwegian Bokmål), "nl" (Dutch), "pl" (Polish), "pt-BR" (Portuguese: Brazil), "pt-PT" (Portuguese: Portugal), "ro" (Romanian), "ru" (Russian), "sk" (Slovak), "sl" (Slovenian), "sr" (Serbian), "sv" (Swedish), "sw" (Swahili), "th" (Thai), "tl" (Tagalog), "tr" (Turkish), "uk" (Ukrainian), "vi" (Vietnamese), "zh" (Chinese), "zh-CN" (Chinese: China): MissingTranslation [warning]
  <string name="VariationsRestrictParameterTitle">Set the restriction on the fetching of the Variations seed</string>
 

Looks like this is intentional:
https://codereview.chromium.org/1062213002

So we just need to suppress these.